阅读说明：本技术 具有辅助层的自旋转移矩mram及其操作方法 (Spin transfer torque MRAM with auxiliary layer and method of operating the same ) 是由 Q.乐 Z.李 Z.白 P.冯德海杰登 M.霍 于 2019-08-30 设计创作，主要内容包括：一种MRAM设备包括磁性隧道结,该磁性隧道结包含：具有固定磁化方向的参考层、自由层和位于参考层与自由层之间的非磁性隧道阻挡层；一个或多个辅助层；和第一非磁性间隔层,该第一非磁性间隔层位于该自由层与该一个或多个辅助层之间。(An MRAM device includes a magnetic tunnel junction, the magnetic tunnel junction comprising: a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer; one or more auxiliary layers; and a first nonmagnetic spacer layer between the free layer and the one or more auxiliary layers.)

1. An MRAM device, the device comprising:

a magnetic tunnel junction comprising a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer;

a negative magnetic anisotropy auxiliary layer having a negative magnetic anisotropy that provides in-plane magnetization in a plane perpendicular to the fixed magnetization direction; and

a first nonmagnetic spacer layer between the free layer and the negative magnetic anisotropy auxiliary layer.

2. The MRAM device of claim 1, wherein:

the negative magnetic anisotropy auxiliary layer has an easy magnetization plane perpendicular to the fixed magnetization of the reference layer; and is

The negative magnetic anisotropy auxiliary layer does not have an easy axis direction in the easy magnetization plane.

3. The MRAM device of claim 1, wherein:

the free layer has positive magnetic anisotropy to provide bistable magnetization states including a parallel state having a magnetization parallel to the fixed vertical magnetization and an anti-parallel state having a magnetization anti-parallel to the fixed vertical magnetization; and is

The magnetic energy of the negative magnetic anisotropy auxiliary layer is invariant under rotation of the magnetization of the negative magnetic anisotropy auxiliary layer in the horizontal plane.

4. The MRAM device of claim 1, wherein the negative magnetic anisotropy assist layer comprises a uniform negative magnetic anisotropy material.

5. The MRAM device of claim 4, wherein the negative magnetic anisotropy assist layer comprises a cobalt-iridium alloy or a cobalt-iron alloy.

6. The MRAM device of claim 5, wherein the negative magnetic anisotropy assist layer comprises the cobalt-iridium alloy comprising 70 at% to 90 at% cobalt and 10 at% to 30 at% iridium.

7. The MRAM device of claim 5, wherein the negative magnetic anisotropy auxiliary layer comprises the cobalt-iron alloy comprising 90 atomic% to 99.5 atomic% cobalt and 0.5 atomic% to 10 atomic% iron.

8. The MRAM device of claim 1, wherein the negative magnetic anisotropy assist layer comprises a multilayer stack comprising a plurality of repetitions of a first magnetic material layer and a second magnetic material layer.

9. The MRAM device of claim 8, wherein:

the first magnetic material layer comprises cobalt; and is

The second magnetic material layer includes iron.

10. The MRAM device of claim 1, further comprising a synthetic antiferromagnetic structure including the reference layer, a fixed ferromagnetic layer having a magnetization antiparallel to the fixed magnetization direction of the reference layer, and an antiferromagnetic coupling layer located between the reference layer and the fixed ferromagnetic layer.

11. The MRAM device of claim 1, further comprising:

a pinned magnetization layer having a positive uniaxial magnetic anisotropy providing a magnetization direction parallel or anti-parallel to the fixed magnetization direction of the reference layer; and

a second nonmagnetic spacer layer located between the negative magnetic anisotropy assist layer and the pinned magnetization layer.

12. The MRAM apparatus of claim 11, wherein a combination of the magnetization of the pinned magnetization layer and the magnetization of the negative magnetic anisotropy assist layer applies a magnetic field to the magnetization of the free layer that is not parallel and perpendicular to the fixed magnetization direction of the reference layer.

13. The MRAM device of claim 1, further comprising a nonmagnetic capping layer located above the negative magnetic anisotropy assist layer.

14. A method of operating an MRAM device, the method comprising:

providing an MRAM device according to claim 1; and

flowing a current through the magnetic tunnel junction, the first nonmagnetic spacer layer, and the negative magnetic anisotropy auxiliary layer.

15. The method of claim 14, wherein the negative magnetic anisotropy auxiliary layer provides an initial torque to the magnetization of the free layer during an initial phase of precession of the magnetization of the free layer about a vertical axis parallel to the fixed magnetization direction of the reference layer when the current begins to flow through the MRAM device.

16. The MRAM device of claim 15, wherein:

during precession of the magnetization of the free layer about a vertical axis parallel to the fixed magnetization direction of the reference layer, coupling occurs between an in-plane magnetization of the negative magnetic anisotropy auxiliary layer and the magnetization of the free layer, and

synchronous precession of the in-plane magnetization of the negative magnetic anisotropy assist layer and the magnetization of the free layer occurs as the current continues to flow through the MRAM device.

17. The method of claim 14, wherein:

the negative magnetic anisotropy auxiliary layer has an easy magnetization plane perpendicular to the fixed magnetization of the reference layer; and is

The negative magnetic anisotropy auxiliary layer does not have an easy axis direction in the easy magnetization plane.

18. The method of claim 14, wherein the negative magnetic anisotropy auxiliary layer comprises a cobalt-iridium alloy or a cobalt-iron alloy.

19. The method of claim 14, wherein the negative magnetic anisotropy auxiliary layer comprises a multilayer stack comprising a plurality of repetitions of a first magnetic material layer and a second magnetic material layer.

20. The method of claim 19, wherein:

the first magnetic material layer consists essentially of cobalt; and is

The second magnetic material layer consists essentially of iron.

21. An MRAM device, the device comprising:

a magnetic tunnel junction comprising a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer;

a first magnetic auxiliary layer;

a second magnetic auxiliary layer;

an antiferromagnetically-coupled spacer layer between the first magnetic assist layer and the second magnetic assist layer, wherein the antiferromagnetically-coupled spacer layer is configured to provide antiferromagnetic coupling between a first magnetization direction of the first magnetic assist layer and a second magnetization direction of the second magnetic assist layer; and

a first nonmagnetic spacer layer between the free layer and the first magnetic assist layer.

22. The MRAM cell of claim 21, wherein the first magnetization direction and the second magnetization direction are configured to precess about a vertical axis parallel to the fixed magnetization direction of the reference layer while maintaining antiferromagnetic alignment therebetween upon application of a current through the first magnetic assist layer, the antiferromagnetic coupling spacer layer, and the second magnetic assist layer.

23. The MRAM cell of claim 22, wherein:

the free layer has positive magnetic anisotropy to provide bistable magnetization states including a parallel state having a magnetization parallel to the fixed vertical magnetization and an anti-parallel state having a magnetization anti-parallel to the fixed vertical magnetization; and is

The fixed magnetization direction of the reference layer maintains the same orientation when the current is applied through the reference layer.

24. The MRAM cell of claim 21, wherein:

the first magnetization direction is a first in-plane magnetization perpendicular to the fixed magnetization direction of the reference layer; and is

The second magnetization direction is a second in-plane magnetization perpendicular to the fixed magnetization direction of the reference layer.

25. The MRAM cell of claim 21, wherein:

the first magnetic auxiliary layer comprises a first magnetic material having a first negative magnetic anisotropy; and is

The second magnetic auxiliary layer comprises a second magnetic material having a second negative magnetic anisotropy.

26. The MRAM cell of claim 25, wherein each of the first and second magnetic assist layers is selected from the group consisting of:

a uniform negative magnetic anisotropy material; and

a multilayer stack comprising a plurality of repetitions of a first magnetic material layer and a second magnetic material layer.

27. The MRAM cell of claim 26, wherein each of the first and second magnetic assist layers comprises a cobalt iridium alloy comprising 70 to 90 at% cobalt and 10 to 30 at% iridium.

28. The MRAM device of claim 26, wherein each of the first and second magnetic assist layers comprises a cobalt-iron alloy comprising 90 atomic% to 99.5 atomic% cobalt and 0.5 atomic% to 10 atomic% iron.

29. The MRAM cell of claim 25, wherein:

at least one of the first and second magnetic assist layers comprises a multilayer stack comprising a periodically repeating unit layer stack; and is

The cell layer stack includes the first magnetic material layer and the second magnetic material layer.

30. The MRAM cell of claim 21, wherein each of the first and second magnetic assist layers comprises a respective magnetic material that does not have uniaxial magnetic anisotropy.

31. The MRAM cell of claim 30, wherein each of the first and second magnetic assist layers comprises a respective material selected from a CoFe alloy or a NiFe alloy having greater than 40 atomic% iron.

32. The MRAM cell of claim 2, wherein the antiferromagnetically-coupled spacer layer comprises a metallic material that causes a Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling interaction between the first and second magnetic auxiliary layers.

33. The MRAM cell of claim 32, wherein the antiferromagnetically-coupled spacer layer comprises ruthenium and has a thickness in a range of 0.1nm to 1.0 nm.

34. The MRAM cell of claim 2, further comprising a synthetic antiferromagnetic structure comprising the reference layer, a fixed ferromagnetic layer having a magnetization antiparallel to the fixed magnetization direction of the reference layer, and an antiferromagnetic coupling layer located between the reference layer and the fixed ferromagnetic layer.

35. The MRAM cell of claim 21, further comprising:

a pinned magnetization layer having a positive uniaxial magnetic anisotropy providing a magnetization direction parallel or anti-parallel to the fixed magnetization direction of the reference layer; and

a second nonmagnetic spacer layer located between the negative magnetic anisotropy assist layer and the pinned magnetization layer.

36. The MRAM cell of claim 21, further comprising a nonmagnetic capping layer, the nonmagnetic capping layer being located above the second magnetic assist layer.

37. The MRAM cell of claim 21, wherein a combination of the first magnetic assist layer, the antiferromagnetically-coupled spacer layer, and the second magnetic assist layer is configured to reduce a switching current magnitude by at least 20% as compared to the same MRAM cell lacking the first magnetic assist layer, the antiferromagnetically-coupled spacer layer, and the second magnetic assist layer.

38. The MRAM cell of claim 21, wherein the MRAM cell is configured to provide magnetic coupling between a magnetization of the free layer and the first magnetization of the first magnetic assist layer during precession of the magnetization of the free layer about a vertical axis parallel to the fixed magnetization direction of the reference layer, and to provide synchronous precession of the first magnetization of the first magnetic assist layer and the magnetization of the free layer when a current flows through the MRAM cell.

39. A method of operating an MRAM cell, the method comprising:

providing an MRAM device according to claim 21; and

a switching current is flowed through the magnetic tunnel junction, the first nonmagnetic spacer layer, and the negative magnetic anisotropy auxiliary layer to switch a spin state of the free layer.

40. The method of claim 39, wherein the switching current magnitude is reduced by at least 20% compared to the same MRAM cell lacking the first magnetic assist layer, the antiferromagnetically-coupled spacer layer, and the second magnetic assist layer.

41. An MRAM cell, the MRAM cell comprising:

a magnetic tunnel junction comprising a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer;

a spin torque oscillator stack; and

a first nonmagnetic spacer layer between the free layer and the spin torque oscillator stack.

42. The MRAM cell of claim 41, wherein the spin torque oscillator stack comprises a spin torque layer, a spin polarizing layer, and a second nonmagnetic spacer located between the spin torque layer and the spin polarizing layer.

43. The MRAM cell of claim 42, wherein the spin torque layer is located on the first nonmagnetic spacer layer, the second nonmagnetic spacer layer is located on the spin torque layer, and the spin polarizing layer is located on the second nonmagnetic spacer layer.

44. The MRAM cell of claim 42, wherein:

the spin-torque layer comprises a first magnetic material having a first tapered magnetization with respect to a vertical axis parallel to the fixed vertical magnetization direction of the reference layer; and is

The spin polarizing layer includes a second magnetic material having a second tapered magnetization relative to the vertical axis parallel to the fixed vertical magnetization direction of the reference layer.

45. The MRAM cell of claim 44, wherein an in-plane component of the first tapered magnetization and an in-plane component of the second tapered magnetization are free to precess about the vertical axis upon application of a current through the spin torque layer, the second nonmagnetic spacer layer, and the spin polarizing layer.

46. The MRAM cell of claim 45, wherein the fixed vertical magnetization direction of the reference layer remains in the same orientation when a current is applied through the reference layer.

47. The MRAM cell of claim 44, wherein the spin polarization layer comprises a ferromagnetic material having the second tapered magnetization relative to the vertical axis.

48. The MRAM cell of claim 47, wherein the ferromagnetic material has an axial magnetization component that is parallel or anti-parallel to an axial magnetization component of the first tapered magnetization of the spin-torque layer.

49. The MRAM cell of claim 44, wherein the spin polarizing layer comprises:

a first spin-polarized component layer having a zero or negative magnetization; and

a second spin-polarized component layer having an axial magnetization parallel or anti-parallel to the vertical axis.

50. The MRAM cell of claim 49, wherein the second spin-polarized component is vertically spaced from the first spin-polarized component layer by a third nonmagnetic spacer layer.

51. The MRAM cell of claim 49, wherein the second spin-polarization component comprises a multilayer stack of a cobalt layer and at least one of a platinum layer or a palladium layer.

52. The MRAM cell of claim 51, wherein the first spin-polarized component comprises a cobalt-iridium alloy comprising 70 atomic% to 90 atomic% cobalt and 10 atomic% to 30 atomic% iridium.

53. The MRAM device of claim 51, wherein the first spin-polarized component comprises a cobalt-iron alloy comprising 90 atomic% to 99.5 atomic% cobalt and 0.5 atomic% to 10 atomic% iron.

54. The MRAM device of claim 51, wherein the first spin-polarized component comprises a CoFeB alloy.

55. The MRAM device of claim 51, wherein the first spin-polarization component comprises a multilayer stack of cobalt and iron layers.

56. The MRAM cell of claim 51, wherein a combined magnetization of the first spin-polarized component layer and the second spin-polarized component layer provides the second taper magnetization coupled to the first taper magnetization of the spin-torque layer.

57. The MRAM cell of claim 42, wherein:

the spin-torque layer includes a first magnetic material having a negative magnetization; and is

The spin polarizing layer includes a second magnetic material having a second tapered magnetization relative to a vertical axis parallel to the fixed vertical magnetization direction of the reference layer.

58. The MRAM cell of claim 57, wherein the first magnetic material comprises a cobalt-iridium alloy comprising 70 atomic% to 90 atomic% cobalt and 10 atomic% to 30 atomic% iridium, a cobalt-iron alloy comprising 90 atomic% to 99.5 atomic% cobalt and 0.5 atomic% to 10 atomic% iron, a cobalt-iron alloy, a cobalt-iron-boron alloy, or a multilayer stack of cobalt and iron layers.

59. The MRAM cell of claim 41, further comprising a non-magnetic capping layer located over the spin polarizing layer and comprising a metallic material.

60. The MRAM cell of claim 41, wherein the spin torque oscillator stack is configured to provide an initial non-vertical torque to a magnetization of the free layer during an initial phase of precession of the magnetization of the free layer about a vertical axis parallel to the fixed vertical magnetization direction of the reference layer when a current begins to flow through the MRAM cell.

Technical Field

The present disclosure relates generally to the field of magnetic memory devices, and in particular to Spin Transfer Torque (STT) Magnetoresistive Random Access Memory (MRAM) devices having an auxiliary layer and methods of operating the same.

Background

Spin transfer torque ("STT") refers to the effect of the orientation of a magnetic layer in a magnetic tunnel junction or spin valve being modified by a spin-polarized current. Generally, the current is unpolarized, wherein the electrons have random spin orientations. Spin polarized current is a current in which electrons have a non-zero net spin due to a distribution of preferential spin orientations. Spin polarized current may be generated by passing a current through a magnetically polarized layer. When a spin-polarized current flows through the free layer of a magnetic tunnel junction or spin valve, electrons in the spin-polarized current may transfer at least some of their angular momentum to the free layer, thereby generating a torque to magnetize the free layer. When a sufficient amount of spin-polarized current passes through the free layer, a spin-transfer torque can be employed to flip the spin orientation (e.g., change magnetization) in the free layer. The difference in resistance of the magnetic tunnel junctions between different magnetization states of the free layer may be exploited to store data within a Magnetoresistive Random Access Memory (MRAM) cell, depending on whether the magnetization of the free layer is parallel or anti-parallel to the magnetization of the reference layer.

Disclosure of Invention

According to one aspect of the present disclosure, an MRAM device includes a magnetic tunnel junction, the magnetic tunnel junction including: a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer; a negative magnetic anisotropy auxiliary layer having a negative magnetic anisotropy that provides an in-plane magnetization in a plane perpendicular to the fixed magnetization direction; and a first nonmagnetic spacer layer between the free layer and the negative magnetic anisotropy auxiliary layer.

According to another aspect of the present disclosure, an MRAM device includes a magnetic tunnel junction including a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer; a first magnetic auxiliary layer; a second magnetic auxiliary layer; an antiferromagnetically coupled spacer layer between the first magnetic assist layer and the second magnetic assist layer; and a first nonmagnetic spacer layer between the free layer and the first magnetic assist layer. The antiferromagnetically-coupled spacer layer is configured to provide antiferromagnetic coupling between a first magnetization direction of the first magnetic assist layer and a second magnetization direction of the second magnetic assist layer.

According to another aspect of the present disclosure, an MRAM cell includes a magnetic tunnel junction including a reference layer having a fixed magnetization direction, a free layer, and a non-magnetic tunnel barrier layer between the reference layer and the free layer; a spin torque oscillator stack; and a first nonmagnetic spacer layer between the free layer and the spin torque oscillator stack.

Drawings

FIG. 1 is a schematic diagram of a memory device including the resistive memory cells of the present disclosure in an array configuration.

Fig. 2 shows a first configuration of an exemplary STT MRAM cell according to a first embodiment of the present disclosure.

Fig. 3 shows a second configuration of an exemplary STT MRAM cell in accordance with a first embodiment of the present disclosure.

Fig. 4 shows a third configuration of an exemplary STT MRAM cell in accordance with the first embodiment of the present disclosure.

Fig. 5 shows a fourth configuration of an exemplary STT MRAM cell in accordance with the first embodiment of the present disclosure.

FIG. 6A illustrates an exemplary precession pattern during a spin transition of the free layer from an upward state to a downward state.

FIG. 6B illustrates an exemplary precession pattern during a spin transition of the free layer from a downward state to an upward state.

Fig. 7 shows a first configuration of an exemplary STT MRAM cell according to a second embodiment of the present disclosure.

Fig. 8 shows a second configuration of an exemplary STT MRAM cell according to a second embodiment of the present disclosure.

Fig. 9 shows a third configuration of an exemplary STT MRAM cell in accordance with a second embodiment of the present disclosure.

Fig. 10 shows a fourth configuration of an exemplary STT MRAM cell in accordance with a second embodiment of the present disclosure.

Fig. 11 shows a comparative STT MRAM cell.

Fig. 12 is a graph showing transition probability as a function of current density by comparing STT MRAM cells.

Fig. 13 is a graph showing transition probability as a function of current density through an exemplary STT MRAM cell of the second embodiment of the present disclosure.

Fig. 14 shows a first configuration of an exemplary STT MRAM cell according to a third embodiment of the present disclosure.

FIG. 15 illustrates antiferromagnetic coupling between the magnetizations of the spin torque layer and the spin polarizing layer within an exemplary spin-transfer torque MRAM cell in a first coupling mode.

FIG. 16 illustrates the antiferromagnetic coupling between the magnetizations of the spin torque layer and the spin polarizing layer within an exemplary spin-transfer torque MRAM cell in the second coupling mode.

FIG. 17 illustrates the antiferromagnetic coupling between the magnetizations of the spin torque layer and the spin polarizing layer within an exemplary spin-transfer torque MRAM cell in the third coupling mode.

FIG. 18 illustrates the antiferromagnetic coupling between the magnetizations of the spin torque layer and the spin polarizing layer within an exemplary spin-transfer torque MRAM cell in the fourth coupling mode.

Fig. 19 shows a second configuration of an exemplary MRAM cell in accordance with a third embodiment of the present disclosure.

Fig. 20 shows a third configuration of an exemplary MRAM cell in accordance with a third embodiment of the present disclosure.

Fig. 21 illustrates a fourth configuration of an exemplary MRAM cell in accordance with a third embodiment of the present disclosure.

Detailed Description

As discussed above, the present disclosure relates to a spin-transfer torque (STT) MRAM device having an auxiliary layer and a method of operating the same, various aspects of which are described below.

The figures are not drawn to scale. Where a single instance of an element is illustrated, multiple instances of the element may be repeated unless explicitly described or otherwise clearly indicated to be absent repetition of the element. The same reference numerals refer to the same or similar elements. Elements having the same reference number are assumed to have the same material composition unless explicitly stated otherwise. Ordinal numbers such as "first," "second," and "third" are used merely to identify similar elements, and different ordinal numbers may be employed throughout the specification and claims of the present disclosure. As used herein, a first element that is positioned "on" a second element may be positioned on the outside of the surface of the second element or on the inside of the second element. As used herein, a first element is "directly" positioned on a second element if there is physical contact between a surface of the first element and a surface of the second element. As used herein, an "in-process" structure or a "transient" structure refers to a structure that is subsequently modified.

As used herein, "layer" refers to a portion of a material that includes a region having a thickness. A layer may extend over the entirety of an underlying or overlying structure, or may have a range that is less than the range of an underlying or overlying structure. In addition, a layer may be a region of uniform or non-uniform continuous structure having a thickness less than the thickness of the continuous structure. For example, a layer may be positioned between the top and bottom surfaces of the continuous structure or between any pair of horizontal planes at the top and bottom surfaces of the continuous structure. The layers may extend horizontally, vertically, and/or along a tapered surface. The substrate may be a layer, may include one or more layers therein, and/or may have one or more layers thereon, above, and/or below.

As used herein, "stack of layers" refers to a stack of layers. As used herein, "line" or "line structure" refers to a layer having a predominant direction of extension, i.e., having the direction in which the layer extends the most.

Referring to FIG. 1, a schematic diagram of a magnetic memory device including a memory cell 180 of an embodiment of the present disclosure in an array configuration is shown. The magnetic memory device may be configured as an MRAM device 500 that contains MRAM cells 180. As used herein, "MRAM device" refers to a memory device that contains cells that allow random access, e.g., any selected memory cell is accessed according to a command for reading the contents of the selected memory cell.

The MRAM device 500 of embodiments of the present disclosure includes a memory array region 550 that includes an array of respective MRAM cells 180 located at intersections of respective word lines (which may include the first electrically conductive lines 30 as shown or the second electrically conductive lines 90 in an alternative configuration) and bit lines (which may include the second electrically conductive lines 90 as shown or the first electrically conductive lines 30 in an alternative configuration). MRAM device 500 may also include a row decoder 560 connected to the word lines, a sense circuit 570 (e.g., sense amplifiers and other bit line control circuitry) connected to the bit lines, a column decoder 580 connected to the bit lines, and a data buffer 590 connected to the sense circuit. Multiple instances of MRAM cell 180 are provided in an array configuration to form MRAM device 500. Thus, each MRAM cell 180 may be a two-terminal device including a respective first electrode and a respective second electrode. It should be noted that the location and interconnection of elements is schematic and that elements may be arranged in different configurations. Furthermore, MRAM cell 180 may be fabricated as a discrete device, i.e., a single isolation device.

Each MRAM cell 180 includes a magnetic tunnel junction or spin valve having at least two different resistance states depending on the alignment of the magnetization of the different magnetic material layers. A magnetic tunnel junction or spin valve is disposed between the first and second electrodes within each MRAM cell 180. The configuration of MRAM cell 180 is described in detail in subsequent sections.

Referring to fig. 2, a first configuration of an exemplary STT MRAM cell 180 of the first embodiment is schematically illustrated. The sttram cell 180 includes a Magnetic Tunnel Junction (MTJ) 140. The magnetic tunnel junction 140 includes a reference layer 132 having a fixed vertical magnetization, a non-magnetic tunnel barrier layer 134 located between the reference layer 132 and a free layer 136. In one implementation, the reference layer 132 is located below the non-magnetic tunnel barrier 134, while the free layer 136 is located above the non-magnetic tunnel barrier 134. However, in other embodiments, the reference layer 132 is located above the non-magnetic tunnel barrier 134 and the free layer 136 is located below the non-magnetic tunnel barrier 134, or the reference layer 132 and the free layer 136 may be located on opposite lateral sides of the non-magnetic tunnel barrier 134. In one embodiment, the reference layer 132 and the free layer 136 have respective positive uniaxial magnetic anisotropy.

In general, the magnetic energy per unit volume of the magnetic thin film depends on the magnetization orientation of the magnetic material of the magnetic thin film. The magnetic energy per unit volume can be determined by the angle θ (or sin) between the magnetization direction and the vertical axis perpendicular to the plane of the magnetic film, such as the top or bottom surface of the magnetic film²θ) and a polynomial of the azimuth angle between the magnetization direction and a fixed vertical plane perpendicular to the plane of the magnetic thin film. As sin²The first and second terms of magnetic energy per unit volume of the function of theta include K₁sin²θ+K₂sin⁴Theta. When K is₁Is negative and K₂Less than-K₁At 2 time_，Function K₁sin²θ+K₂sin⁴Theta has a minimum value when theta is pi/2. If the energy of the magnetic anisotropy as a function of theta has a minimum only when theta is pi/2, the magnetization of the magnetic film is preferablyRemains entirely within the film plane and the film is said to have "negative magnetic anisotropy". If the magnetic anisotropy energy as a function of θ has a minimum only when θ is 0 or π, then the magnetization of the magnetic film is perpendicular to the film plane and the film is said to have "positive magnetic anisotropy". A thin crystal magnetic film having positive magnetic anisotropy has a tendency that magnetization remains perpendicular to the plane of the thin crystal magnetic film (i.e., perpendicular to both directions in which the thin crystal magnetic film laterally extends). A thin crystal magnetic film having negative magnetic anisotropy has magnetization in the plane of the thin crystal magnetic film, although magnetization in the film plane does not have a preferred orientation.

The configuration of the reference layer 132 and the free layer 136 with corresponding positive uniaxial magnetic anisotropy provides a bistable magnetization state for the free layer 136. The bistable magnetization states include a parallel state in which the free layer 136 has a magnetization (e.g., magnetization direction) that is parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer 132, and an anti-parallel state in which the free layer 136 has a magnetization (e.g., magnetization direction) that is anti-parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer 132.

The reference layer 132 may include a Co/Ni or Co/Pt multilayer structure. The reference layer 132 may also include a thin nonmagnetic layer composed of tantalum having a thickness of 0.2nm to 0.5nm and a thin CoFeB layer (thickness in the range of 0.5nm to 3 nm). The non-magnetic tunnel barrier layer 134 may comprise any tunneling barrier material, such as an electrically insulating material, for example, magnesium oxide. The thickness of the non-magnetic tunnel barrier layer 134 may be 0.7nm to 1.3nm, such as about 1 nm. The free layer 136 may include an alloy of one or more of Fe, Co, and/or Ni, such as CoFeB whose composition provides positive uniaxial magnetic anisotropy.

In one embodiment, the reference layer 132 may be provided as a component within a synthetic antiferromagnetic structure (SAF structure) 120. The SAF structure 120 may include: a reference layer 132; a fixed ferromagnetic layer 112 having a magnetization antiparallel to the fixed vertical magnetization; and an antiferromagnetic coupling layer 114 located between the reference layer 132 and the fixed ferromagnetic layer 112, the fixed ferromagnetic layer facing a first side of the reference layer 132 opposite a second side of the reference layer 132, the second side facing the non-magnetic tunnel barrier layer 134. The thickness of the antiferromagnetic coupling layer 114 causes antiferromagnetic coupling between the reference layer 132 and the fixed ferromagnetic layer 112. In other words, the antiferromagnetic coupling layer 114 can lock the antiferromagnetic alignment between the magnetization of the reference layer 132 and the magnetization of the fixed ferromagnetic layer 112 to lock the magnetization of the reference layer 132 and the magnetization of the fixed ferromagnetic layer 112 in place. In one embodiment, the antiferromagnetic coupling layer may include ruthenium and may have a thickness in the range of 0.3nm to 1 nm.

The first nonmagnetic spacer layer 150 is disposed over a second side of the free layer 136 opposite the first side of the free layer 136 facing the nonmagnetic tunnel barrier layer 134. First nonmagnetic spacer layer 150 comprises a nonmagnetic material such as tantalum, ruthenium, tantalum nitride, copper nitride, or magnesium oxide. In one embodiment, the first nonmagnetic spacer layer 150 may comprise a conductive metal material. Alternatively, the first nonmagnetic spacer layer 150 may include a tunneling dielectric material such as magnesium oxide. The thickness of the first nonmagnetic spacer layer 150 may be in the range of 0.2nm to 2nm, although lesser and greater thicknesses may also be used.

A negative magnetic anisotropy assist layer 160 may be disposed over the first nonmagnetic spacer layer 150 and over the second side of the free layer 136. The negative magnetic anisotropy auxiliary layer 160 may have a sufficiently negative value K₁To provide in-plane magnetization to the negative magnetic anisotropy assist layer 160. The in-plane magnetization is the magnetization that lies in the horizontal plane in fig. 2, which is perpendicular to the fixed vertical magnetization of the reference layer 132.

In one embodiment, the hard axis of magnetization is parallel to the normal direction of the major surface of the negative magnetic anisotropy assist layer 160 (i.e., the axis is perpendicular to the plane of layer 160 and parallel to the fixed vertical magnetization of reference layer 132), while the easy magnetization plane is parallel to the plane of the negative magnetic anisotropy assist layer 160 (i.e., the easy magnetization plane is perpendicular to the fixed vertical magnetization of reference layer 132 in fig. 2). In one embodiment, in the plane of the negative magnetic anisotropy auxiliary layer 160 (i.e., the easy magnetization plane), there is no easy axis direction. The negative magnetic anisotropy assist layer 160 is spin-coupled with the free layer 136 through the first nonmagnetic spacer layer 150.

In one embodiment, the azimuthal dependence of the magnetic anisotropy of the negative magnetic anisotropy auxiliary layer 160The property component may be zero or equal to the thermal energy at room temperature (i.e., k)_BT, wherein k_BT is 297.15 kelvin (which is room temperature)) is less significant than boltzmann's constant. For example, the maximum change in magnetic anisotropy per unit volume around a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 may be less than 1/2 times the thermal energy at room temperature. In such cases, upon application of a current through the negative magnetic anisotropy assist layer 160, the magnetization of the negative magnetic anisotropy assist layer 160 is free to precess in a horizontal plane parallel to the interface between the first nonmagnetic spacer layer 150 and the negative magnetic anisotropy assist layer 160. In one embodiment, the magnetic energy of the negative magnetic anisotropy auxiliary layer 160 may be constant under a rotation of the magnetization of the negative magnetic anisotropy auxiliary layer 160 in a horizontal plane.

In one embodiment, the negative magnetic anisotropy assist layer 160 comprises a uniform negative magnetic anisotropy material. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. In one embodiment, the negative magnetic anisotropy assist layer 160 includes and/or consists essentially of a cobalt-iridium alloy. The material composition of the cobalt iridium alloy may be selected to provide negative magnetic anisotropy. In one embodiment, the cobalt-iridium alloy may include cobalt atoms at an atomic concentration in the range of 60% to 98% (such as 70% to 90%, for example 80%) and iridium atoms at an atomic concentration in the range of 40% to 2% (such as 30% to 10%, for example 20%). In one embodiment, the cobalt-iridium alloy includes only cobalt, iridium, and unavoidable impurities. In another embodiment, up to 5 atomic percent of an element other than cobalt and iridium may be added to the alloy. In the illustrative example, with Co_0.8Ir_0.2K of cobalt iridium alloy₁A value of about-0.6 × 10⁶J/m³. In another embodiment, the negative magnetic anisotropy auxiliary layer 160 includes and/or consists essentially of a cobalt-iron alloy. The material composition of the cobalt-iron alloy may be selected to provide negative magnetic anisotropy. In one embodiment, the cobalt-iron alloy may include cobalt atoms in an atomic concentration in the range of 80% to 99.8% (such as 90% to 99.5%, e.g., 99%) and 20% to 0.2% (such as 10% to 0.5%, e.g., 1%)An iron atom within the range of (1). In the illustrative example, with Co_0.9Fe_0.1K of cobalt-iron alloy of composition₁A value of about-0.99 × 10⁶J/m³. The thickness of the negative magnetic anisotropy assist layer 160 may be in the range of 1nm to 10nm, such as 1.5nm to 6nm, although lesser and greater thicknesses may also be employed.

In one implementation, the non-magnetic capping layer 170 may be located over the negative magnetic anisotropy assist layer 160. The non-magnetic capping layer 170 may comprise a non-magnetic, conductive material such as W, Ti, Ta, WN, TiN, TaN, Ru, and Cu. The thickness of the non-magnetic capping layer 170 may be in the range of 1nm to 20nm, although lesser and greater thicknesses may also be employed.

The layer stack comprising the material layers from the SAF structure 120 to the nonmagnetic capping layer 170 may be deposited up or down, i.e., from the SAF structure 120 toward the nonmagnetic capping layer 170 or from the nonmagnetic capping layer 170 toward the SAF structure 120. The layer stack may be formed as a stack of continuous layers, and then a discrete patterned layer stack of each MRAM cell 180 may be patterned.

MRAM cell 180 may include a first terminal 92 electrically connected to or including a portion of bit line 90 (shown in fig. 1) and a second terminal 32 electrically connected to or including a portion of word line 30 (shown in fig. 1). The positions of the first and second terminals may be switched such that the first terminal is electrically connected to the SAF structure 120 and the second terminal is electrically connected to the capping layer 170.

Alternatively, each MRAM cell 180 may include a dedicated steering device, such as an access transistor or diode configured to activate the respective discrete patterned layer stack (120,140,150,160,170) when a suitable voltage is applied to the steering device. The steering device may be electrically connected between the patterned layer stack and one of the respective word line 30 or bit line 90 of the respective MRAM cell 180.

In one embodiment, the polarity of the voltage applied to the first terminal 92 may be changed depending on the polarity of the magnetization state to be programmed in the free layer 136. For example, a voltage of a first polarity may be applied to the first terminal 92 (relative to the second terminal 32) during a transition from an anti-parallel state to a parallel state, and a voltage of a second polarity (which is opposite the first polarity) may be applied to the first terminal 92 during a transition from a parallel state to an anti-parallel state. Further, circuit variations for activating the discrete patterned layer stack (120,140,150,160,170) are also contemplated herein.

By flowing a current through the stack of discrete patterned layers (120,140,150,160,170), the magnetization direction of the free layer 136 can be flipped (i.e., from up to down, or vice versa). The magnetization of the free layer 136 may precess around the vertical direction (i.e., the direction of flow of current) during the programming process until the direction of magnetization reverses 180 degrees, at which time the flow of current stops. In one embodiment, the magnetization of the negative magnetic anisotropy assist layer 160 is free to rotate about a vertical axis parallel to the fixed magnetization direction of the reference layer 132 while a current flows through the stack of discrete patterned layers (120,140,150,160,170). This configuration allows the negative magnetic anisotropy assist layer 160 to provide an initial non-vertical torque to the magnetization of the free layer 136 during an initial phase of precession of the magnetization of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 when a current begins to flow through the MRAM cell 180.

In one embodiment, the MRAM cell 180 may be configured to provide coupling between the in-plane magnetization of the negative magnetic anisotropy assist layer 160 and the magnetization of the free layer 136 during precession of the magnetization of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization of the reference layer 132, and to provide synchronous precession of the in-plane magnetization of the negative magnetic anisotropy assist layer 160 and the magnetization of the free layer 136 when a current is passed through the MRAM cell 180.

Due to the negative magnetic anisotropy of the negative magnetic anisotropy assist layer 160, in one embodiment, the in-plane magnetization of the negative magnetic anisotropy assist layer 160 may provide an initial torque to the free layer to facilitate the onset of switching of the free layer 136. Once the free layer 136 begins precession, the free layer 136 may provide a spin torque to the negative magnetic anisotropy assist layer 160 to cause the negative magnetic anisotropy assist layer 160 magnetization to also precess. This precession of the negative magnetic anisotropy assist layer 160 may in turn further assist in the switching of the free layer 136. Embodiments in which the negative magnetic anisotropy auxiliary layer 160 has an in-plane easy magnetization plane and lacks a fixed easy axis direction are more effective than prior art auxiliary layers in which the magnetization direction (e.g., easy axis) of the auxiliary layer is fixed.

Referring to fig. 3, a second configuration of the exemplary spin-transfer torque MRAM cell 180 may be derived from the first configuration of the exemplary spin-transfer torque MRAM cell 180 of fig. 2 by replacing the negative magnetic anisotropy assist layer 160, which has a uniform material composition, with a negative magnetic anisotropy assist layer 260 that includes a multilayer stack (262, 264). The multilayer stack (262,264) may include multiple repetitions of the first magnetic material layer 262 and the second magnetic material layer 264. First magnetic material layer 262 may include and/or may consist essentially of a first magnetic material. The second magnetic material layer 264 may include and/or may consist essentially of a second magnetic material.

The composition and thickness of each first magnetic material layer 262 and the composition and thickness of each second magnetic material layer 264 may be selected such that the multilayer stack (262,264) provides an in-plane magnetization, i.e., a magnetization perpendicular to the fixed magnetization direction of the reference layer 132 (i.e., an easy magnetization plane perpendicular to the fixed magnetization direction of the reference layer 132 and having no easy axis). The negative magnetic anisotropy auxiliary layer 260 may have a sufficiently negative value K₁To provide in-plane magnetization to the negative magnetic anisotropy auxiliary layer 260.

In one embodiment, the azimuthal-dependent component of the magnetic anisotropy of the negative magnetic anisotropy auxiliary layer 260 may be zero or insignificant compared to thermal energy at room temperature. For example, the maximum change in magnetic anisotropy per unit volume around a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 may be less than 1/2 times the thermal energy at room temperature. In such cases, upon application of a current through the negative magnetic anisotropy auxiliary layer 260, the magnetization of the negative magnetic anisotropy auxiliary layer 260 is free to precess in a plane parallel to the interface between the first nonmagnetic spacer layer 150 and the negative magnetic anisotropy auxiliary layer 260. In one embodiment, the magnetic energy of the negative magnetic anisotropy auxiliary layer 260 may be constant under the magnetization of the negative magnetic anisotropy auxiliary layer 260 rotating in the horizontal plane.

In one embodiment, first magnetic material layer 262 includes cobalt,and the second magnetic material layer 264 includes iron. In one embodiment, first magnetic material layer 262 consists essentially of cobalt and second magnetic material layer 264 consists essentially of iron. The thickness of each first magnetic material layer 262 may be in the range of 0.3nm to 1nm, and the thickness of each second magnetic material layer 264 may be in the range of 0.3nm to 1 nm. The total number of repetitions within the negative magnetic anisotropy auxiliary layer 260 (i.e., the total number of pairs of the first and second magnetic material layers 262 and 264) may be in a range of 2 to 20, such as 4 to 10. In one embodiment, the multilayer stack (262,264) includes a periodically repeating unit layer stack including a first magnetic material layer 262 and a second magnetic material layer 264. In illustrative examples, K comprising a repeating cobalt-iron multilayer stack of a unit layer stack consisting of cobalt and iron layers having the same thickness₁The value may be about-1.1 × 10⁶J/m³。

Referring to FIG. 4, a third configuration of the example spin-transfer torque MRAM cell 180 may be derived from the first configuration of the example spin-transfer torque MRAM cell 180 of FIG. 2 by interposing a second nonmagnetic spacer layer 190 and a pinned magnetization layer 192 between the negative magnetic anisotropy assist layer 160 and the nonmagnetic capping layer 170.

The second nonmagnetic spacer layer 190 may be located on the negative magnetic anisotropy auxiliary layer 160 on the opposite side of the first nonmagnetic spacer layer 150. The second nonmagnetic spacer layer 190 comprises a nonmagnetic material such as tantalum, ruthenium, tantalum nitride, copper nitride, or magnesium oxide. In one embodiment, the second nonmagnetic spacer layer 190 may comprise a conductive material. Alternatively, the second nonmagnetic spacer layer 190 may include a dielectric material such as magnesium oxide. The thickness of the second nonmagnetic spacer layer 190 may be in the range of 0.2nm to 2nm, although lesser and greater thicknesses may also be used. The second nonmagnetic spacer layer 190 may comprise the same material as the first nonmagnetic spacer layer 150 or may comprise a different material than the first nonmagnetic spacer layer.

The pinned magnetization layer 192 is a magnetic layer having positive uniaxial magnetic anisotropy. In other words, K₁Is positive, and K₁sin²Theta dominates all other higher order terms and dependencesThe term of sin (n Π) (or cos (n Π)) of magnetic anisotropy energy per volume of material in pinned magnetization layer 192. The positive uniaxial magnetic anisotropy of the pinned magnetization layer 192 provides a magnetization that is parallel or antiparallel to the fixed vertical magnetization of the reference layer 132. In one embodiment, K of pinned magnetization layer 192₁The value may be greater than K of the free layer 136₁The value is such that the magnetization of pinned magnetization layer 192 remains pinned in the vertical direction (i.e., perpendicular to the interface between the various layers of the discrete patterned layer stack (120,140,150,160,190,192,170)) during programming of MRAM cell 180. The magnetization of the pinned magnetization layer 192 may remain parallel or anti-parallel to the magnetization of the reference layer 132.

In one embodiment, the pinned magnetization layer 192 may comprise a Co/Ni or Co/Pt multilayer structure. The pinned magnetization layer 192 may also include a thin nonmagnetic layer composed of tantalum having a thickness of 0.2nm to 0.5nm and a thin CoFeB layer (thickness in the range of 0.5nm to 3 nm). The pinned magnetization layer 192 may cause the in-plane magnetization of the negative magnetic anisotropy assist layer 160 to oscillate. Oscillation of the in-plane magnetization of the negative magnetic anisotropy assist layer 160 may generate a rotating spin torque on the magnetization of the free layer 136 during programming, and thus may help switch the magnetization of the free layer 136 with a lower current through the discrete patterned layer stack (120,140,150,160,190,192,170). In one implementation, the combination of the magnetizations of the pinned magnetization layer 192 and the negative magnetic anisotropy assist layer 160 applies non-horizontal and non-vertical magnetic fields (i.e., fields that are neither parallel nor perpendicular to the magnetization direction of the reference layer 132) to the magnetization of the free layer 136 to reduce the required magnitude of current through the discrete patterned layer stack (120,140,150,160,190,192,170) during switching of the magnetization of the free layer 136.

Referring to fig. 5, a fourth configuration of an exemplary spin-transfer torque MRAM cell 180 may be derived from the third configuration of the exemplary spin-transfer torque MRAM cell 180 of fig. 4 by replacing the negative magnetic anisotropy assist layer 160, having a uniform material composition, with a negative magnetic anisotropy assist layer 260 comprising a multi-layer stack (262,264) that includes multiple repetitions of the first and second magnetic material layers 262,264 described above with respect to fig. 3.

Referring to all configurations of the exemplary spin-transfer torque MRAM cell 180 shown in FIGS. 1-5, the exemplary spin-transfer torque MRAM cell 180 can be individually programmed and read. Reading (i.e., sensing) the magnetization state of the free layer 136 may be performed by applying a read bias voltage across the first terminal 92 and the second terminal 32 of the selected discrete patterned layer stack {120,140,150, (160 or 260),170} or {120,140,150, (160 or 260), (190,192),170 }. The parallel or anti-parallel alignment between the magnetization of the free layer 136 and the reference layer 132 determines the resistance of the selected discrete patterned layer stack in each MRAM cell 180, and thus the magnitude of the current flowing between the first terminal 92 and the second terminal 32. The magnitude of the current may be sensed to determine the magnetization state of the free layer 136 and the data encoded by the detected magnetization state.

Programming of the opposite magnetization state of the exemplary spin-transfer torque MRAM cell 180 to the free layer 136 may be performed by flowing a current through a selected discrete patterned layer stack {120,140,150, (160 or 260),170} or {120,140,150, (160 or 260), (190,192),170} and by causing the magnetization direction of the free layer 136 to flip (i.e., switch). Specifically, current can flow through a selected discrete patterned layer stack that includes the magnetic tunnel junction 140, the first nonmagnetic spacer layer 150, and the negative magnetic anisotropy assist layer (160 or 260). As current begins to flow through the magnetic tunnel junction 140, the first nonmagnetic spacer layer 150, and the negative magnetic anisotropy assist layer (160 or 260), the in-plane magnetization of the negative magnetic anisotropy assist layer (160 or 260) provides an initial non-vertical torque to the magnetization of the free layer 136 during an initial phase of precession of the magnetization of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization of the reference layer 132.

In one embodiment, during precession of the magnetization of the free layer 136 about the Vertical Axis (VA) parallel to the fixed vertical magnetization of the reference layer 132, the in-plane magnetization of the negative magnetic anisotropy assist layer (160 or 260) couples with the magnetization of the free layer 136 to provide synchronous precession of the in-plane magnetization M2 of the negative magnetic anisotropy assist layer (160 or 260) and the magnetization M1 of the free layer 136 while current flows through the MRAM cell 180, as shown in fig. 6A and 6B. FIG. 6A shows the transition of the magnetization M1 of the free layer 136 from the "up" state to the "down" state, and FIG. 6B shows the transition of the magnetization M1 of the free layer 136 from the "down" state to the "up" state. In one implementation, the in-plane magnetization M2 of the negative magnetic anisotropy assist layer (160 or 260) and the magnetization M1 of the free layer 136 may remain in the same rotational vertical plane during switching of the magnetization of the free layer 136. The coupling between the horizontal (in-plane) component of the magnetization M1 of the free layer 136 and the in-plane magnetization M2 of the negative magnetic anisotropy assist layer (160 or 260) may be antiferromagnetic or ferromagnetic. Fig. 6A and 6B show an example in which the coupling between the horizontal (in-plane) component of the magnetization M1 of the free layer 136 and the in-plane magnetization M2 of the negative magnetic anisotropy assist layer (160 or 260) during switching of the magnetization of the free layer 136 is antiferromagnetic.

Fig. 7 shows a STT MRAM cell 180 according to a second embodiment. The layers 112, 114, 132, 134, 136 and 150 of the STT MRAM cell 180 of the second embodiment may be the same as the respective layers 112, 114, 132, 134, 136 and 150 of the STT MRAM cell 180 of the first embodiment shown in fig. 2, and thus are described above with respect to the first embodiment. A first magnetic assist layer 162 may be disposed on the first nonmagnetic spacer layer 150. The first magnetic auxiliary layer 162 includes a first magnetic material having a first magnetic anisotropy. In one implementation, the first magnetic auxiliary layer 162 may have a sufficiently negative value of K₁To provide a first in-plane magnetization to the first magnetic assist layer 162. The in-plane magnetization is a magnetization that lies in a horizontal plane perpendicular to the fixed vertical magnetization direction of the reference layer 132.

In one embodiment, the azimuthal-dependent component of the magnetic anisotropy of the first magnetic auxiliary layer 162 may be zero or equal to thermal energy at room temperature (i.e., k)_BT, wherein k_BT is 297.15 kelvin (which is room temperature)) is less significant than boltzmann's constant. For example, the maximum variation of magnetic anisotropy energy per unit volume around a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 may be less than 1/2 times the thermal energy at room temperature. In such cases, upon application of a current through the first magnetic assist layer 162, the magnetization of the first magnetic assist layer 162 is free to precess in a horizontal plane parallel to the interface between the first nonmagnetic spacer layer 150 and the first magnetic assist layer 162. At one isIn an embodiment, the magnetic energy of the first magnetic auxiliary layer 162 may be constant under the rotation of the magnetization of the first magnetic auxiliary layer 162 in the horizontal plane.

In one embodiment, a material with negative magnetic anisotropy, such as the first magnetic assist layer 162, has a hard axis of magnetization that is parallel to the normal direction of the major surfaces of the layers (i.e., the axis is perpendicular to the plane of the layers and parallel to the fixed vertical magnetization direction of the reference layer 132), while an easy plane of magnetization is parallel to the plane of the layers (i.e., the easy plane of magnetization is perpendicular to the fixed vertical magnetization direction of the reference layer 132 in fig. 7). In one embodiment, there is no easy axis direction in the easy magnetization plane.

In one embodiment, the first magnetic assist layer 162 includes a uniform negative magnetic anisotropy material. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. In one embodiment, the first magnetic assist layer 162 includes and/or consists essentially of a cobalt iridium alloy. The material composition of the cobalt iridium alloy may be selected to provide negative magnetic anisotropy. In one embodiment, the cobalt-iridium alloy may include cobalt atoms at an atomic concentration in the range of 60% to 98% (such as 70% to 90%, for example 80%) and iridium atoms at an atomic concentration in the range of 40% to 2% (such as 30% to 10%, for example 20%). In one embodiment, the cobalt-iridium alloy includes only cobalt, iridium, and unavoidable impurities. In another embodiment, up to 5 atomic percent of an element other than cobalt and iridium may be added to the alloy. In the illustrative example, with Co_0.8Ir_0.2K of cobalt iridium alloy₁A value of about-0.6 × 10⁶J/m³. In another embodiment, the first magnetic auxiliary layer 162 includes and/or consists essentially of a cobalt iron alloy having a hexagonal crystal structure. The material composition of the cobalt-iron alloy may be selected to provide negative magnetic anisotropy. In one embodiment, the cobalt-iron alloy may include cobalt atoms at an atomic concentration in the range of 80% to 99.8% (such as 90% to 99.5%, e.g., 99%) and iron atoms at an atomic concentration in the range of 20% to 0.2% (such as 10% to 0.5%, e.g., 1%). In the illustrative example, with Co_0.9Fe_0.1K of cobalt-iron alloy of composition₁A value of about-0.99 × 10⁶J/m³. The thickness of the first magnetic auxiliary layer 162 may be in the range of 1nm to 10nm, such as 1.5nm to 6nm, although lesser and greater thicknesses may also be employed.

In another embodiment, the first magnetic assist layer 162 comprises a multi-layer stack including multiple repetitions of the first and second magnetic material layers. The first magnetic material layer may include and/or may consist essentially of the first magnetic material. The second magnetic material layer may include and/or may consist essentially of a second magnetic material. The composition and thickness of each first magnetic material layer and the composition and thickness of each second magnetic material layer may be selected such that the multilayer stack provides in-plane magnetization, i.e., magnetization perpendicular to the fixed magnetization direction of the reference layer 132. The first magnetic auxiliary layer 162 may have a sufficiently negative value K₁To provide a first in-plane magnetization to the first magnetic assist layer 162.

In one embodiment, the first magnetic material layer includes cobalt and the second magnetic material layer includes iron. In one embodiment, the first magnetic material layer consists essentially of cobalt and the second magnetic material layer consists essentially of iron. The thickness of each first magnetic material layer may be in a range of 0.3nm to 1nm, and the thickness of each second magnetic material layer may be in a range of 0.3nm to 1 nm. The total number of repetitions within the first magnetic assist layer 162 (i.e., the total number of pairs of the first and second magnetic material layers) may be in a range of 2 to 20, such as 4 to 10. In one embodiment, the multilayer stack includes a periodically repeating unit layer stack including a first magnetic material layer and a second magnetic material layer.

The antiferromagnetically-coupled spacer layer 164 may be located between a first magnetic assist layer and a second magnetic assist layer, such as on the first magnetic assist layer 162 on opposite sides of the first nonmagnetic spacer layer 150, which is located between the free layer 136 and the first magnetic assist layer 162. The antiferromagnetic coupling spacer layer 164 includes a metallic material that induces a Ruderman-Kittel-Kasuya-yosida (rkky) coupling interaction between the first magnetic assist layer 152 and a second magnetic assist layer 166, which in one embodiment is located on the antiferromagnetic coupling spacer layer 164. In the RKKY coupling interaction, the local internal d-or f-shell electron spins defining the magnetization direction of a ferromagnetic metal layer interact through conduction electrons in an intermediate non-magnetic material layer to define the direction of a preferred magnetization direction in another ferromagnetic metal layer. The thickness of the antiferromagnetically-coupled spacer layer 164 can be selected such that the direction of magnetization in the second plane of the second magnetic assist layer 166 is antiparallel to the direction of magnetization in the first plane of the first magnetic assist layer 162. In other words, the thickness of the antiferromagnetic coupling spacer layer 164 can be in a range that provides antiferromagnetic coupling between the first magnetization direction of the first magnetic assist layer 162 and the second magnetization direction of the second magnetic assist layer 166. In one embodiment, the antiferromagnetic coupling spacer layer 164 includes or consists essentially of ruthenium and has a thickness in the range of 0.1nm to 1.0 nm.

A second magnetic assist layer 166 may be disposed on the antiferromagnetically-coupled spacer layer 164. The second magnetic assist layer 166 includes a second magnetic material having a second magnetic anisotropy, which may be the same as or different from the material of the first magnetic assist layer 162. In one implementation, the second magnetic auxiliary layer 166 may have a sufficiently negative value of K₁To provide a second in-plane magnetization direction to the second magnetic assist layer 166. The in-plane magnetization direction is a magnetization direction that lies in a horizontal plane perpendicular to the fixed vertical magnetization direction of the reference layer 132.

In one embodiment, the azimuthal-dependent component of the magnetic anisotropy of second magnetic auxiliary layer 166 may be zero or equal to thermal energy at room temperature (i.e., k)_BT, wherein k_BT is 297.15 kelvin (which is room temperature)) is less significant than boltzmann's constant. For example, the maximum change in magnetic anisotropy per unit volume around a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 may be less than 1/2 times the thermal energy at room temperature. In such cases, the magnetization of the second magnetic assist layer 166 is parallel to the antiferromagnetic coupling when a current is applied through the second magnetic assist layer 166The interface between the spacer layer 164 and the second magnetic assist layer 166 is free to precess in the horizontal plane. In one embodiment, the magnetic energy of the second magnetic assist layer 166 may be invariant under rotation of the second magnetic assist layer 166 in a horizontal plane.

In one embodiment, the second magnetic assist layer 166 includes a uniform negative magnetic anisotropy material. In one embodiment, the second magnetic assist layer 166 includes and/or consists essentially of a cobalt-iridium alloy or a cobalt-iron alloy as described with respect to the first magnetic assist layer 162. The material composition of the cobalt iridium alloy may be selected to provide negative magnetic anisotropy. In one embodiment, the cobalt-iridium alloy may include cobalt atoms at an atomic concentration in the range of 20% to 80% and iridium atoms at the remaining atomic concentration. The thickness of the second magnetic assist layer 166 may be in the range of 1nm to 10nm, such as 1.5nm to 6nm, although lesser and greater thicknesses may also be employed.

In another embodiment, the second magnetic assist layer 166 includes a multi-layer stack including multiple repetitions of the first magnetic material layer and the second magnetic material layer. The first magnetic material layer may include and/or may consist essentially of the first magnetic material. The second magnetic material layer may include and/or may consist essentially of a second magnetic material. The composition and thickness of each first magnetic material layer and the composition and thickness of each second magnetic material layer may be selected such that the multilayer stack provides in-plane magnetization, i.e., magnetization perpendicular to the fixed magnetization direction of the reference layer 132. The second magnetic auxiliary layer 166 may have a sufficiently negative value of K₁To provide a second in-plane magnetization to the second magnetic assist layer 166.

In one embodiment, the first magnetic material layer includes cobalt and the second magnetic material layer includes iron. In one embodiment, the first magnetic material layer consists essentially of cobalt and the second magnetic material layer consists essentially of iron. The thickness of each first magnetic material layer may be in a range of 0.3nm to 1nm, and the thickness of each second magnetic material layer may be in a range of 0.3nm to 1 nm. The total number of repetitions within the second magnetic assist layer 166 (i.e., the total number of pairs of the first and second magnetic material layers) may be in the range of 2 to 20, such as 4 to 10. In one embodiment, the multilayer stack includes a periodically repeating unit layer stack including a first magnetic material layer and a second magnetic material layer.

In general, each of the first and second magnetic assist layers 162,166 may be independently selected from a uniform negative magnetic anisotropy material, and a multi-layer stack comprising multiple repetitions of first and second magnetic material layers. In one embodiment, each of the first and second magnetic assist layers 162,166 may be independently selected from a cobalt iridium alloy, a cobalt iron alloy having a hexagonal crystal structure and low iron content, or a multi-repeated multilayer stack comprising a stack of units of cobalt and iron layers. In one embodiment, at least one of the first and second magnetic assist layers 162,166 includes a multilayer stack including a periodically repeating unit layer stack, and the unit layer stack includes first and second magnetic material layers.

In one implementation, the non-magnetic capping layer 170 may be located over the second magnetic assist layer 166. The non-magnetic capping layer 170 may comprise a non-magnetic, conductive material such as W, Ti, Ta, WN, TiN, TaN, Ru, and Cu. The thickness of the non-magnetic capping layer 170 may be in the range of 1nm to 20nm, although lesser and greater thicknesses may also be employed.

The layer stack comprising the material layers from the SAF structure 120 to the nonmagnetic capping layer 170 may be deposited up or down, i.e., from the SAF structure 120 toward the nonmagnetic capping layer 170 or from the nonmagnetic capping layer 170 toward the SAF structure 120. The layer stack may be formed as a stack of continuous layers, and then a discrete patterned layer stack of each MRAM cell 180 may be patterned.

MRAM cell 180 may include a first terminal 92 electrically connected to or including a portion of bit line 90 (shown in fig. 1) and a second terminal 32 electrically connected to or including a portion of word line 30 (shown in fig. 1). The positions of the first and second terminals may be switched such that the first terminal is electrically connected to the SAF structure 120 and the second terminal is electrically connected to the capping layer 170.

Alternatively, each MRAM cell 180 may include a dedicated steering device, such as an access transistor or diode configured to activate the respective discrete patterned layer stack (120,140,150,162,164,166,170) when a suitable voltage is applied to the steering device. The steering device may be electrically connected between the patterned layer stack and one of the respective word line 30 or bit line 90 of the respective MRAM cell 180.

In one embodiment, the polarity of the voltage applied to the first terminal 92 may be changed depending on the polarity of the magnetization state to be programmed in the free layer 136. For example, a voltage of a first polarity may be applied to the first terminal 92 (relative to the second terminal 32) during a transition from an anti-parallel state to a parallel state, and a voltage of a second polarity (which is opposite the first polarity) may be applied to the first terminal 92 during a transition from a parallel state to an anti-parallel state. Further, circuit variations for activating the discrete patterned layer stack (120,140,150,162,164,166,170) are also contemplated herein.

By flowing a current through the stack of discrete patterned layers (120,140,150,162,164,166,170), the magnetization direction of the free layer 136 can be flipped (i.e., from up to down, or vice versa). The magnetization direction of the free layer 136 may precess around the vertical direction (i.e., the direction of flow of current) during the programming process until the direction of magnetization reverses 180 degrees, at which time the flow of current stops.

The first magnetization direction of the first magnetic assist layer 162 and the second magnetization direction of the second magnetic assist layer 166 are free to precess about a fixed vertical magnetization direction parallel to the reference layer 132 while maintaining antiferromagnetic alignment therebetween upon application of current through the first magnetic assist layer 162, the antiferromagnetic coupling spacer layer 164, and the second magnetic assist layer 166, e.g., during programming. The fixed vertical magnetization direction of the reference layer 132 remains in the same orientation when a current is applied through the reference layer 132.

During operation of the magnetic storage device, current may flow through the magnetic tunnel junction 140, the first nonmagnetic spacer layer 150, the first magnetic assist layer 162, the antiferromagnetic coupling spacer layer 164, and the second magnetic assist layer 166.

In one embodiment, the first magnetic assist layer 162, the antiferromagnetically-coupled spacer layer 164, and the second magnetic assist layer 166 help to keep the electron spins of the free layer more in-plane to counteract the spin torque that tilts the electron spins out-of-plane. Due to the antiferromagnetic coupling, the antiferromagnetically coupled auxiliary film comprising the first magnetic auxiliary layer 162, the antiferromagnetically coupled spacer layer 164, and the second magnetic auxiliary layer 166 also tends to be a single domain within each layer, thereby maintaining a more uniform magnetization during the process of switching the auxiliary free layer 136, which is more desirable. An additional benefit of this embodiment is that flux closure within the tri-layer auxiliary film can minimize stray fields from the antiferromagnetically coupled auxiliary film on the free layer 136, which will help improve the thermal stability and data retention of the MRAM cell 180.

In one embodiment, the combination of the first magnetic assist layer 162, the antiferromagnetic coupling spacer layer 164, and the second magnetic assist layer 166 is configured to provide an initial non-vertical torque to the magnetization of the free layer 136 during an initial phase of precession of the magnetization of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization direction of the reference layer 132 when a current begins to flow through the MRAM cell 180. The MRAM cell 180 is configured to provide magnetic coupling between the magnetization direction of the free layer 136 and the first magnetization direction of the first magnetic assist layer 162 during precession of the magnetization direction of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization direction of the reference layer 132, and to provide synchronous precession of the first magnetization direction of the first magnetic assist layer 162 and the magnetization direction of the free layer 136 when a current flows through the MRAM cell 180.

Referring to fig. 8, a second configuration of the exemplary spin-transfer torque MRAM cell 180 may be derived from the first configuration of the exemplary spin-transfer torque magnetic memory device shown in fig. 7 by replacing the first magnetic assist layer 162 having a first in-plane magnetization with a first magnetic assist layer 263 comprising a first ferromagnetic material that does not have uniaxial magnetic anisotropy, and by replacing the second magnetic assist layer 166 having a second in-plane magnetization with a second magnetic assist layer 266 comprising a second ferromagnetic material that does not have uniaxial magnetic anisotropy. In one embodiment, the first magnetic assist layer 263 and the second magnetic assist layer 266 may have a non-uniaxial magnetic anisotropy. As used herein, "non-uniaxial magnetic anisotropy" refers to a magnetic anisotropy in which the minimum value of magnetic anisotropy energy per volume does not occur in the direction of θ ═ 0, θ ═ pi, or θ ═ pi/2 for all values of Φ. In other words, the orientation of magnetization in a magnetic film having non-uniaxial magnetic anisotropy is not perpendicular to the plane of the magnetic film or the vertical direction of the set of all in-plane directions.

The thickness of the antiferromagnetic coupling spacer layer 164 is selected to provide antiferromagnetic coupling between the first magnetization of the first magnetic assist layer 263 and the second magnetization of the second magnetic assist layer 266. Thus, the first magnetization of the first magnetic assist layer 263 and the second magnetization of the second magnetic assist layer 266 may be antiferromagnetically coupled. Furthermore, the change in energy of magnetic anisotropy per volume as a function of the spatial orientation of the first and second magnetizations (which remain antiparallel to each other) may be comparable to or less than the thermal energy at room temperature (i.e., k)_BT, wherein T is 293.15 Kelvin).

Each of the first and second magnetic auxiliary layers 263,266 comprises a respective soft magnetic material that does not have a uniaxial magnetic anisotropy, which may be the same or different. In one embodiment, each of the first and second magnetic auxiliary layers 263,266 comprises and/or consists essentially of a respective material selected from: a CoFe alloy having greater than 40 atomic% iron (such as 45 to 70 atomic% iron) and the balance cobalt, or a NiFe alloy.

By flowing a current through the stack of discrete patterned layers (120,140,150,263,164,266,170), the magnetization direction of the free layer 136 can be flipped (i.e., from up to down, or vice versa). The magnetization direction of the free layer 136 may precess around the vertical direction (i.e., the direction of flow of current) during the programming process until the direction of the magnetization direction reverses 180 degrees, at which point the flow of current stops.

The magnetization direction of the free layer 136 can be programmed by flowing a current through the discrete patterned layer stack (120,140,150,263,164,266,170), for example, from a parallel state parallel to the fixed vertical magnetization direction of the reference layer 132 to an anti-parallel state anti-parallel to the fixed magnetization direction of the reference layer 132, or vice versa. The first magnetization direction of the first magnetic auxiliary layer 263 and the second magnetization direction of the second magnetic auxiliary layer 266 are centered around a vertical axis parallel to the fixed vertical magnetization direction of the reference layer 132

Free to precess at an angle between 0 and 180 degrees relative to a vertical axis while maintaining antiferromagnetic alignment therebetween upon application of current through the first magnetic assist layer 263, the antiferromagnetic coupling spacer layer 164 and the second magnetic assist layer 266, e.g., during programming. The tilt angle of the first magnetization direction of the first magnetic assist layer 263 and the second magnetization direction of the second magnetic assist layer 266 during programming is synchronized with the angle of the magnetization direction of the free layer 136 when the tilt angle changes from 0 degrees to 180 degrees or from 180 degrees to 0 degrees relative to the vertical axis during programming of the MRAM cell 180. The fixed vertical magnetization direction of the reference layer 132 remains in the same orientation when a current is applied through the reference layer 132.

During operation of the magnetic memory device, current may flow through the magnetic tunnel junction 140, the first nonmagnetic spacer layer 150, the first magnetic assist layer 263, the antiferromagnetic coupling spacer layer 164, and the second magnetic assist layer 266. The combination of the first magnetic assist layer 263, the antiferromagnetically-coupled spacer layer 164, and the second magnetic assist layer 266 is configured to provide an initial non-vertical torque to the magnetization direction of the free layer 136 during an initial phase of precession of the magnetization direction of the free layer 136 about a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer 132 when a current begins to flow through the MRAM cell 180. The MRAM cell 180 is configured to provide magnetic coupling between the magnetization direction of the free layer 136 and the first magnetization direction of the first magnetic assist layer 263 during precession of the magnetization direction of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization direction of the reference layer 132, and to provide synchronous precession of the first magnetization direction of the first magnetic assist layer 263 and the magnetization direction of the free layer 136 when a current flows through the MRAM cell 180.

Referring to FIG. 9, a third configuration of the example spin-transfer torque MRAM cell 180 may be derived from the first configuration of the example spin-transfer torque MRAM cell 180 of FIG. 7 by interposing a second nonmagnetic spacer layer 190 and a pinned magnetization layer 192 between the second magnetic assist layer 166 and the nonmagnetic capping layer 170.

The second nonmagnetic spacer layer 190 may be located on the second magnetic assist layer 166 on the opposite side of the antiferromagnetic coupling spacer layer 164. The second nonmagnetic spacer layer 190 comprises a nonmagnetic material such as tantalum, ruthenium, tantalum nitride, copper nitride, or magnesium oxide. In one embodiment, the second nonmagnetic spacer layer 190 may comprise a conductive material. Alternatively, the second nonmagnetic spacer layer 190 may include a tunneling dielectric material such as magnesium oxide. The thickness of the second nonmagnetic spacer layer 190 may be in the range of 0.2nm to 2nm, although lesser and greater thicknesses may also be used. The second nonmagnetic spacer layer 190 may comprise the same material as the first nonmagnetic spacer layer 150 or may comprise a different material than the first nonmagnetic spacer layer.

The pinned magnetization layer 192 is a magnetic layer having positive uniaxial magnetic anisotropy. In other words, K₁Is positive, and K₁sin²θ dominates all other higher order terms and the term of sin (n φ) (or cos (n φ)) which depends on the magnetic anisotropy energy per volume of the material of the pinned magnetization layer 192. The positive uniaxial magnetic anisotropy of the pinned magnetization layer 192 provides a magnetization that is parallel or antiparallel to the fixed vertical magnetization of the reference layer 132. In one embodiment, K of pinned magnetization layer 192₁The value may be greater than K of the free layer 136₁The value is such that the magnetization of pinned magnetization layer 192 remains pinned in the vertical direction (i.e., perpendicular to the interface between the various layers of the discrete patterned layer stack (120,140,150,162,164,166,190,192,170)) during programming of MRAM cell 180. The magnetization of the pinned magnetization layer 192 may remain parallel or anti-parallel to the magnetization of the reference layer 132.

In one embodiment, the pinned magnetization layer 192 may comprise a Co/Ni or Co/Pt multilayer structure. The pinned magnetization layer 192 may also include a thin nonmagnetic layer composed of tantalum with a thickness of 0.2nm to 0.5nm and a thin CoFeB layer (thickness in the range of 0.5nm to 3n m). The pinned magnetization layer 192 may cause the in-plane magnetization of the second magnetic assist layer 166 to oscillate. The out-of-plane oscillation of the magnetization of the second magnetic assist layer 166 may generate a rotating spin torque on the magnetization of the free layer 136 during programming, and thus may help switch the magnetization of the free layer 136 with a smaller current through the discrete patterned layer stack (120,140,150,162,164,166,190,192,170). In one implementation, the magnetization combination of the pinned magnetization layer 192, the first magnetic assist layer 162, and the second magnetic assist layer 166 applies a non-horizontal, non-vertical magnetic field (i.e., a field neither parallel nor perpendicular to the fixed magnetization direction of the reference layer 132) to the magnetization of the free layer 136 to reduce the required magnitude of current through the discrete patterned layer stack (120,140,150,162,164,166,190,192,170) during switching of the magnetization of the free layer 136.

Referring to fig. 10, a fourth configuration of the exemplary spin-transfer torque MRAM cell 180 may be derived from the third configuration of the exemplary spin-transfer torque magnetic memory device shown in fig. 9 by replacing the first magnetic assist layer 162 having a first in-plane magnetization with a first magnetic assist layer 263 comprising a first ferromagnetic material that does not have uniaxial magnetic anisotropy, and by replacing the second magnetic assist layer 166 having a second in-plane magnetization with a second magnetic assist layer 266 comprising a second ferromagnetic material that does not have uniaxial magnetic anisotropy.

The thickness of the antiferromagnetic coupling spacer layer 164 is selected to provide antiferromagnetic coupling between the first magnetization of the first magnetic assist layer 263 and the second magnetization of the second magnetic assist layer 266. Thus, the first magnetization of the first magnetic assist layer 263 and the second magnetization of the second magnetic assist layer 266 may be antiferromagnetically coupled. Furthermore, the change in energy of magnetic anisotropy per volume as a function of the spatial orientation of the first and second magnetizations (which remain antiparallel to each other) may be comparable to or less than the thermal energy at room temperature (i.e., k)_BT, wherein T is 293.15 Kelvin).

Referring to all configurations of the exemplary spin-transfer torque MRAM cell 180 shown in FIG. 1 and FIGS. 7 through 10, the exemplary spin-transfer torque MRAM cell 180 can be individually programmed and read. Reading (i.e., sensing) the magnetization state of the free layer 136 may be performed by applying a read bias voltage across the first terminal 92 and the second terminal 32 of the selected discrete patterned layer stack {120,140,150, (162 or 263),164, (166 or 266),170} or {120,140,150, (162 or 263),164, (166 or 266), (190,192),170 }. The parallel or anti-parallel alignment between the magnetization of the free layer 136 and the reference layer 132 determines the resistance of the selected discrete patterned layer stack in each MRAM cell 180, and thus the magnitude of the current flowing between the first terminal 92 and the second terminal 32. The magnitude of the current may be sensed to determine the magnetization state of the free layer 136 and the data encoded by the detected magnetization state.

FIG. 11 shows a comparative spin-transfer torque MRAM cell 280 that may be derived from an exemplary spin-transfer torque MRAM cell 180 by omitting all of the magnetic assist layer (162,166,263,266), the antiferromagnetic coupling spacer layer 164, the second nonmagnetic spacer layer 190, and the pinned magnetization layer 192. Thus, assist features during precession of the magnetization of the free layer 136 are not present in the comparative spin-transfer torque MRAM cell 280. The effect of the first and second magnetic assist layers (162,166) and the antiferromagnetically-coupled spacer layer 164 in the first configuration of the exemplary spin-torque-transfer MRAM cell 180 is illustrated in FIGS. 12 and 13.

FIG. 12 is a graph illustrating probability of a transition as a function of current density through the comparative spin-transfer torque magnetic memory device of FIG. 11. FIG. 13 is a graph illustrating probability of a transition as a function of current density through the first configuration of the exemplary spin-transfer torque magnetic memory device illustrated in FIG. 7. The device areas of fig. 12 and 13 are substantially the same.

FIG. 12 shows about 3.9 × 10 for the comparative spin-transfer torque MRAM cell 280 of FIG. 11¹⁰A/m²Is necessary to cause a transition of the magnetization of the free layer 136 from the parallel state to the anti-parallel state within 5 nanoseconds, and is about-2.1 × 10¹⁰A/m²Is necessary to cause a transition in the magnetization of the free layer 136 from an anti-parallel state to a parallel state within 5 nanoseconds fig. 13 illustrates approximately 2.5 × 10 for the exemplary spin-transfer torque MRAM cell 180 of fig. 7¹⁰A/m²Is necessary to cause a transition of the magnetization of the free layer 136 from the parallel state to the anti-parallel state in 5 nanoseconds, and is about-1.68 × 10¹⁰A/m²Is necessary to cause a transition of the magnetization of the free layer 136 from an anti-parallel state to a parallel state within 5 nanoseconds.Thus, FIGS. 12 and 13 illustrate that the presence of the first and second magnetic assist layers 162,166 and the antiferromagnetically-coupled spacer layer 164 in the first configuration of the exemplary spin-transfer torque MRAM cell 180 reduces the current density required for a parallel-to-antiparallel transition (i.e., the magnitude of the switching current) and for an antiparallel-to-parallel transition by 20 to 30% for the first configuration of the exemplary spin-transfer torque MRAM cell 180. Thus, the switching current magnitude of the embodiment MRAM cell 180 is reduced by at least 20% compared to the same MRAM cell 280 lacking the first magnetic assist layer, the antiferromagnetically-coupled spacer layer, and the second magnetic assist layer.

Fig. 14 shows a STT MRAM cell 180 according to a third embodiment. The layers 112, 114, 132, 134 and 136 of the second embodiment of the STT MRAM cell 180 can be the same as the corresponding layers 112, 114, 132, 134 and 136 of the first embodiment of the STT MRAM cell 180 shown in FIG. 2, and thus are described above with respect to the first embodiment. The first nonmagnetic spacer layer 150 is disposed over a second side of the free layer 136 opposite the first side of the free layer 136 facing the nonmagnetic tunnel barrier layer 134. The first nonmagnetic spacer layer 150 comprises a nonmagnetic material. The first nonmagnetic spacer layer 150 may comprise an electrically insulating material (i.e., a dielectric material), such as magnesium oxide. Alternatively, the first nonmagnetic spacer layer 150 may include a conductive metal material, such as tantalum, ruthenium, tantalum nitride, copper, or copper nitride. The thickness of the first nonmagnetic spacer layer 150 may be in the range of 0.2nm to 2nm, although lesser and greater thicknesses may also be used.

A spin-torque layer 362 can be disposed on the first nonmagnetic spacer layer 150. In a first embodiment shown in FIG. 14, the spin-torque layer 362 comprises a first magnetic material having a first tapered magnetization (e.g., magnetization direction) with respect to a vertical direction parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer 132. As used herein, "tapered magnetization" refers to a rotational magnetization (e.g., magnetization direction) having an angle greater than zero and less than 90 degrees, such as 10 to 80 degrees, e.g., 30 to 60 degrees, with respect to an axis parallel to a fixed vertical magnetization (e.g., magnetization direction) of the reference layer 132.

For magnetic anisotropy energy per volume, tapered magnetization can be provided for various symmetry types. For example, the tilt angle θ and azimuth angle φ of a ferromagnetic film with magnetic anisotropy energy per volume to a vertical axis may have the form E/V ═ K₁sin²θ+K₂sin⁴θ+K₃sin⁶θ cos (6 φ), the magnetic anisotropy energy per volume has a six-fold axis of rotational symmetry about a vertical axis perpendicular to the plane of the ferromagnetic film. If K is₁Is negative and K₂Greater than K₁And/2, then the ferromagnetic film has a bi-directional taper of easy magnetization direction at two values of q. Cone angle theta of bidirectional cone easy to magnetize_c1And theta_c2Through theta_c2＝π–θ_c1And (4) correlating.

Ferromagnetic films with different magnetic anisotropy symmetries can provide tapered magnetization in a similar manner. For example, the tilt angle θ and azimuth angle φ of a ferromagnetic film with tetrahedral symmetry per volume of magnetic anisotropy energy to the vertical axis may have the form E/V ═ K₁sin²θ+K₂sin⁴θ+K₃sin⁴Functional dependence of θ sin (2 φ). The inclination angle theta and azimuth angle phi of the ferromagnetic film per volume of magnetic anisotropy energy with rhomboid symmetry to the vertical axis may have the form E/V ═ K₁sin²θ+K₂sin⁴θ+K₃cosθsin³Functional dependence of θ cos (3 φ). If K is₃Is zero or is 1/2k_BT (where k)_BBeing boltzmann's constant, T is room temperature in kelvin, i.e., 293.15 for magnetic anisotropy energy per volume), then the conical magnetization is free to rotate (e.g., oscillate at high frequency) about a vertical axis.

In one embodiment, the azimuthal-dependent component of the magnetic anisotropy of the spin-torque layer 362 may be zero or equal to thermal energy at room temperature (i.e., k)_BT, wherein k_BT is 297.15 kelvin (which is room temperature)) is less significant than boltzmann's constant. For example, the maximum variation of magnetic anisotropy energy per unit volume around a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 may be less than 1/2 times the thermal energy at room temperature. In thatIn this case, when a current is applied through the spin-torque layer 362, the tapered magnetization of the spin-torque layer 362 is free to precess in a horizontal plane parallel to the interface between the first nonmagnetic spacer layer 150 and the spin-torque layer 362. In one embodiment, the magnetic energy of the spin-torque layer 362 may be invariant under rotation of the magnetization of the spin-torque layer 362 in the horizontal plane.

The spin-torque layer 362 can include any ferromagnetic film that provides a tapered magnetization. For example, the spin-torque layer 362 can include a tapered magnetization material, such as a rare earth element (such as neodymium, erbium), or an alloy of at least one rare earth element and a non-rare earth element (such as iron, boron, cobalt, copper, and/or zirconium). In one embodiment, the spin-torque layer 362 can include a uniform tapered magnetization material, i.e., a uniform material that provides tapered magnetization. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. The spin-torque layer 362 can have a thickness in the range of 0.6nm to 10nm, such as 1.2nm to 5nm, although lesser and greater thicknesses can also be used.

The second nonmagnetic spacer layer 364 may be located on the spin torque layer 362 on the opposite side of the first nonmagnetic spacer layer 150. In one embodiment, the second nonmagnetic spacer layer 364 comprises an electrically insulating layer, such as magnesium oxide, having a thickness between 0.2nm and 2 nm.

A spin polarizing layer 366 may be disposed on the second nonmagnetic spacer layer 364. In another embodiment, the order of formation of the spin torque layer 362 and the spin polarizing layer 366 can be reversed such that the spin polarizing layer 366 is located closer to the free layer 136 than the spin torque layer 362. In general, a spin-torque oscillator stack (e.g., an auxiliary layer stack) 370 includes a second nonmagnetic spacer layer 364 between the spin-torque layer 362 and the spin polarizing layer 366.

The spin-polarizing layer 366 has a tapered magnetization, which is referred to herein as a second tapered magnetization. The spin polarizing layer 366 may include a single layer of magnetic material or multiple layers of magnetic material. The second taper magnetization of the spin-polarizing layer 366 may be provided by a single layer of magnetic material having a second taper magnetization, or may be provided by a set of layers of ferromagnetic material having an in-plane magnetization and a vertical (i.e., vertical or axial) magnetization. The second magnetic material has an in-plane magnetization component that is perpendicular to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer 132. The in-plane magnetization component of the second magnetic material is antiferromagnetically coupled to the in-plane component of the first tapered magnetization.

FIG. 14 shows an embodiment in which the spin-polarizing layer 366 consists of a single layer of ferromagnetic material having a second tapered magnetization with respect to a vertical direction parallel to the fixed vertical magnetization of the reference layer 132. In one embodiment, the azimuthal-dependent component of the magnetic anisotropy of spin-polarizing layer 366 may be zero or equal to thermal energy at room temperature (i.e., k)_BT, wherein k_BT is 297.15 kelvin (which is room temperature)) is less significant than boltzmann's constant. For example, the maximum variation of magnetic anisotropy energy per unit volume around a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 may be less than 1/2 times the thermal energy at room temperature. In such cases, when current is applied through the spin polarizing layer 366, the tapered magnetization of the spin polarizing layer 366 is free to precess in a horizontal plane parallel to the interface between the first nonmagnetic spacer layer 150 and the spin torque layer 362. In one embodiment, the magnetic energy of the spin polarizing layer 366 can be invariant under rotation of the magnetization of the spin polarizing layer 366 in the horizontal plane.

In one embodiment, the spin polarizing layer 366 may include any ferromagnetic film that provides a tapered magnetization. For example, the spin-polarizing layer 366 may include a tapered magnetized material, such as a rare earth element (such as neodymium, erbium), or an alloy of at least one rare earth element and a non-rare earth element (such as iron, boron, cobalt, copper, and/or zirconium). In one embodiment, the spin polarizing layer 366 may comprise a uniform taper magnetization material, i.e., a uniform material that provides a taper magnetization. The ferromagnetic materials of the spin torque layer 362 and the spin polarizing layer 366 can be the same or different. The thickness of the spin polarizing layer 366 may be in the range of 0.6nm to 10nm, such as 1.2nm to 5nm, although lesser and greater thicknesses may also be employed.

If the magnetization of the spin polarizing layer 366 is a tapered magnetization, i.e., a second tapered magnetization, the second tapered magnetization of the spin polarizing layer 366 can couple with the first tapered magnetization of the spin torque layer 362 in various modes.

FIG. 15 illustrates a first magnetization M of a spin-torque layer 362 within an exemplary spin-transfer torque MRAM cell 180_TSecond cone magnetization M with spin-polarizing layer 366_PA first mode of antiferromagnetic coupling therebetween. FIG. 15 shows the first magnetization M of the spin-torque layer 362 at the moment of precession_TSecond cone magnetization M with spin-polarizing layer 366_PRelative alignment therebetween. In the first mode, the first taper magnetization M of the spin-torque layer 362_TAnd the first tapered magnetization M of the spin-polarizing layer 366_PMay be anti-parallel to each other.

In this mode, the in-plane component of the first taper magnetization of the spin torque layer 362 and the in-plane magnetization component of the magnetization of the spin polarizing layer 366 (which may be a second taper magnetization) may be antiferromagnetically aligned. In this mode, the in-plane component of the first taper magnetization of the spin torque layer 362 and the in-plane magnetization component of the magnetization of the spin polarizing layer 366 precesses freely about a vertical axis that is parallel to the vertical direction (i.e., parallel to the direction of the fixed vertical magnetization (e.g., magnetization direction) of the reference layer 132 and perpendicular to the various interfaces of the stack (120,140,150,370,170)) while maintaining antiferromagnetic alignment when current is applied through the spin torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366. The fixed vertical magnetization of the reference layer 132 maintains the same orientation when a current is applied through the reference layer 132.

In FIG. 15, the magnetization M of the spin-torque layer 362_TThe magnetization M of the spin-torque layer 362 can be in the horizontal plane (as will be described in more detail with respect to the fourth embodiment shown in FIG. 21 below), or_TMay be tapered and have a taper angle (i.e., a vertical direction perpendicular to the interface between the various material layers and representing the first tapered magnetization direction M)_TAngle between vectors of) and the second tapered magnetization M of the spin-polarizing layer 366_PMay be at the first taper magnetization M of the spin-torque layer 362_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PRemains the same during precession that occurs during programming of the magnetization of the free layer 136 (i.e., during a reversal of the vertical magnetization of the free layer 136 from a parallel state to an anti-parallel state, or vice versa). In one embodiment, the relative angle between the total magnetization (i.e., the second taper magnetization) of the spin polarizing layer 366 and the first taper magnetization of the spin torque layer 362 remains fixed at a value selected from the range of 90 degrees (excluded) to 180 degrees (included) when current is applied through the spin torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366.

FIG. 16 illustrates a first magnetization M of a spin-torque layer 362 within an exemplary spin-transfer torque MRAM cell 180_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PA second mode of antiferromagnetic coupling therebetween. FIG. 16 shows the first magnetization M of the spin-torque layer 362 at the moment of precession_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PRelative alignment therebetween. In the second mode, the first taper magnetization M of the spin-torque layer 362_TAnd a second conical magnetization M of the spin-polarizing layer 366_PMay be parallel to each other, i.e. both may point upwards or both may point downwards.

First taper magnetization M of spin-torque layer 362_TThe angle of taper (i.e. the vertical direction perpendicular to the interface between the various material layers and representing the first taper magnetization M_TAngle between vectors of directions of) and the second taper magnetization M of the spin-polarizing layer 366_PMay be at the first taper magnetization M of the spin-torque layer 362_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PThe precession period of (a) varies. In one embodiment, the relative angle between the total magnetization (e.g., the second taper magnetization) of the spin polarizing layer 366 and the first taper magnetization of the spin torque layer 362 varies in the range of 90 degrees (excluded) to 180 degrees (included) when current is applied through the spin torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366.

FIG. 17 illustrates a first magnetization M of a spin-torque layer 362 within an exemplary spin-transfer torque MRAM cell 180_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PA third mode of antiferromagnetic coupling therebetween. FIG. 17 shows the first magnetization M of the spin-torque layer 362 at the moment of precession_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PBetweenRelative alignment of (a). In the third mode, the first taper magnetization M of the spin-torque layer 362_TAnd a second conical magnetization M of the spin-polarizing layer 366_PMay be anti-parallel to each other, i.e., one pointing upward and the other pointing downward.

First taper magnetization M of spin-torque layer 362_TThe angle of taper (i.e. the vertical direction perpendicular to the interface between the various material layers and representing the first taper magnetization M_TAngle between vectors of directions of) and the second taper magnetization M of the spin-polarizing layer 366_PMay be at the first taper magnetization M of the spin-torque layer 362_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PThe precession period of (a) varies. In one embodiment, the relative angle between the total magnetization (e.g., the second taper magnetization) of the spin polarizing layer 366 and the first taper magnetization of the spin torque layer 362 varies in the range of 90 degrees (excluded) to 180 degrees (included) when current is applied through the spin torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366.

FIG. 18 illustrates a first magnetization M of a spin-torque layer 362 within an exemplary spin-transfer torque MRAM cell 180_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PA fourth mode of antiferromagnetic coupling therebetween. FIG. 18 shows the first magnetization M of the spin-torque layer 362 at the moment of precession_TAnd a second tapered magnetization M of the spin-polarizing layer 366_PRelative alignment therebetween. In the fourth mode, the first taper magnetization M of the spin-torque layer 362_TAnd a second conical magnetization M of the spin-polarizing layer 366_PMay be anti-parallel to each other and the frequencies of the first and second magnetizations are the same such that the first and second magnetization direction vectors point in opposite directions (e.g., when one points to the left, the other points to the right, and vice versa). Thus, in the fourth mode, the first magnetization direction and the second magnetization direction are both anti-parallel in the vertical direction and the horizontal direction (e.g., both the vertical component and the horizontal component of the first tapered magnetization direction and the second tapered magnetization direction are anti-parallel).

This fourth mode causes a large amount of noise and is not preferable compared to the first mode, the second mode, and the third mode. Thus, the first magnetization direction and the second magnetization direction are preferably not anti-parallel in both the vertical direction and the horizontal direction.

Returning to fig. 14, the spin polarizing layer 366 may be provided as a single spin polarizing layer having a uniform composition and may have a second tapered magnetization with respect to the vertical direction. In general, a single spin polarizing layer may have an axial magnetization component that is parallel or anti-parallel to the axial magnetization component of the first tapered magnetization of the spin torque layer 362. In some implementations, the spin polarizing layer 366 can have an axial magnetization component that is antiparallel to the axial component of the tapered magnetization of the spin torque layer 362 (i.e., a vertical magnetization component). In some implementations, the spin polarizing layer 366 can have an axial magnetization component (i.e., a vertical magnetization component) that is parallel to the axial component of the tapered magnetization of the spin torque layer 362.

In one implementation, the nonmagnetic capping layer 170 may be located over the spin polarizing layer 366. The non-magnetic capping layer 170 may comprise a non-magnetic, conductive material such as W, Ti, Ta, WN, TiN, TaN, Ru, and Cu. The thickness of the non-magnetic capping layer 170 may be in the range of 1nm to 20nm, although lesser and greater thicknesses may also be employed.

The layer stack comprising the material layers from the SAF structure 120 to the nonmagnetic capping layer 170 may be deposited up or down, i.e., from the SAF structure 120 toward the nonmagnetic capping layer 170 or from the nonmagnetic capping layer 170 toward the SAF structure 120. The layer stack may be formed as a stack of continuous layers, and then a discrete patterned layer stack of each MRAM cell 180 may be patterned.

MRAM cell 180 may include a first terminal 92 electrically connected to or including a portion of bit line 90 (shown in fig. 1) and a second terminal 32 electrically connected to or including a portion of word line 30 (shown in fig. 1). The positions of the first and second terminals may be switched such that the first terminal is electrically connected to the SAF structure 120 and the second terminal is electrically connected to the capping layer 170.

Alternatively, each MRAM cell 180 may include a dedicated steering device, such as an access transistor or diode configured to activate the respective discrete patterned layer stack (120,140,150,162,164,166,170) when a suitable voltage is applied to the steering device. The steering device may be electrically connected between the patterned layer stack and one of the respective word line 30 or bit line 90 of the respective MRAM cell 180.

In one embodiment, the polarity of the voltage applied to the first terminal 92 may be changed depending on the polarity of the magnetization state to be programmed in the free layer 136. For example, a voltage of a first polarity may be applied to the first terminal 92 (relative to the second terminal 32) during a transition from an anti-parallel state to a parallel state, and a voltage of a second polarity (which is opposite the first polarity) may be applied to the first terminal 92 during a transition from a parallel state to an anti-parallel state. Further, circuit variations for activating the discrete patterned layer stack (120,140,150,162,164,166,170) are also contemplated herein.

By flowing a current through the stack of discrete patterned layers (120,140,150,362,364,366,170), the magnetization direction of the free layer 136 can be flipped (i.e., from up to down, or vice versa). The magnetization direction of the free layer 136 may precess around the vertical direction (i.e., the direction of flow of current) during the programming process until the direction of magnetization reverses 180 degrees, at which time the flow of current stops.

Upon application of current through the spin torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366, the first tapered magnetization of the spin torque layer 362 and the second tapered magnetization of the spin polarizing layer 366 precess freely about a vertical axis parallel to the fixed vertical magnetization of the reference layer 132, e.g., during programming. The fixed vertical magnetization of the reference layer 132 maintains the same orientation when a current is applied through the reference layer 132.

During operation of the MRAM cell, a current may flow through the magnetic tunnel junction 140, the first nonmagnetic spacer layer 150, the spin torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366. The spin-torque oscillator stack 370, which includes the combination of the spin-torque layer 362, the second nonmagnetic spacer layer 364, and the spin polarizing layer 366, is configured to provide an initial non-vertical torque to the magnetization of the free layer 136 during an initial phase of precession of the magnetization of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization of the reference layer 132 when current begins to flow through the MRAM cell 180. The MRAM cell 180 is configured to provide magnetic coupling between the magnetization of the free layer 136 and the first magnetization of the spin-torque layer 362 during precession of the magnetization of the free layer 136 about a vertical axis parallel to the fixed vertical magnetization of the reference layer 132, and to provide synchronous precession of the first magnetization of the spin-torque layer 362 and the magnetization of the free layer 136 when a current flows through the MRAM cell 180.

Referring to fig. 19, a second configuration of an exemplary spin-transfer torque MRAM cell 180 is shown. Spin polarizing layer 366 includes a stacked stack of multiple layers (3662,3664,3666) having different material compositions. The spin polarization layer 366 includes a stacked stack of a first spin polarization component layer 3662 having the same magnetization as the in-plane magnetization component of the second taper magnetization. The first spin-polarized component layer 3662 may have zero or negative uniaxial magnetic anisotropy such that the magnetization of the first spin-polarized component layer 3662 is parallel to the interfaces between the various layers of the MRAM cell 180.

In one embodiment, the first spin-polarized component layer 3662 includes and/or consists essentially of a cobalt-iridium alloy. The material composition of the cobalt iridium alloy may be selected to provide negative uniaxial magnetic anisotropy. In one embodiment, the cobalt-iridium alloy may include cobalt atoms at an atomic concentration in a range of 60% to 98% (such as 70% to 90%), and iridium atoms at a remaining atomic concentration. In the illustrative example, with Co_0.8Ir_0.2K of cobalt iridium alloy₁A value of about-0.6 × 10⁶J/m³. In another embodiment, the first spin-polarized component layer 3662 includes and/or consists essentially of a cobalt-iron alloy. The material composition of the cobalt-iron alloy may be selected to provide negative uniaxial magnetic anisotropy. In one embodiment, the cobalt-iron alloy may include cobalt atoms at an atomic concentration in a range of 80% to 99.8% (such as 90% to 99.5%), and iron atoms at a remaining atomic concentration. In the illustrative example, with Co_0.9Ir_0.1K of cobalt-iron alloy of composition₁A value of about-0.99 × 10⁶J/m³. In another embodiment, the first spin-polarized component layer 3662 includes a cobalt-iron-boron (CoFeB) alloy and/or is substantiallyConsisting of a cobalt iron boron (CoFeB) alloy. The thickness of the first spin-polarized component layer 3662 may be in the range of 1nm to 10nm, such as 1.5nm to 6nm, although lesser and greater thicknesses may also be employed.

In another embodiment, the first spin-polarized component layer 3662 includes a multilayer stack including multiple repetitions of a first magnetic material layer and a second magnetic material layer. The first magnetic material layer may include and/or may consist essentially of the first magnetic material. The second magnetic material layer may include and/or may consist essentially of a second magnetic material.

In one embodiment, the first magnetic material layer includes cobalt and the second magnetic material layer includes iron. In one embodiment, the first magnetic material layer consists essentially of cobalt and the second magnetic material layer consists essentially of iron. The thickness of each first magnetic material layer may be in a range of 0.3nm to 1nm, and the thickness of each second magnetic material layer may be in a range of 0.3nm to 1 nm. The total number of repetitions within the first spin-polarized component layer 3662 (i.e., the total number of pairs of the first magnetic material layer and the second magnetic material layer) may be in the range of 2 to 20, such as 4 to 10. In one embodiment, the multilayer stack includes a periodically repeating unit layer stack including a first magnetic material layer and a second magnetic material layer. In illustrative examples, K comprising a repeating cobalt-iron multilayer stack of a unit layer stack consisting of cobalt and iron layers having the same thickness₁The value may be about-1.1 × 10⁶J/m³。

The spin polarization layer 366 also includes a second spin-polarized component layer 3666 having an axial magnetization that is parallel or anti-parallel to the vertical direction of the reference layer 132. In one embodiment, the second spin-polarized component layer 3666 includes a multilayer stack of cobalt layers and platinum or palladium layers. The second spin-polarized component layer 3666 may have positive uniaxial magnetic anisotropy such that the magnetization of the second spin-polarized component layer 3666 is axial, i.e., perpendicular to the interfaces between the various layers of the MRAM cell 180. The axial magnetization of the second spin-polarized component layer 3666 may be parallel or anti-parallel to the fixed vertical magnetization direction of the reference layer 132.

In one embodiment, the second spin-polarized component layer 3666 may be vertically spaced apart from the first spin-polarized component layer 3662 by an optional third nonmagnetic spacer layer 3664. Third nonmagnetic spacer layer 3664 may include a nonmagnetic material such as MgO, Cu, Ag, AgSn, Cr, or Ge. In one embodiment, the first spin-polarized component layer 3662 may be in contact with the second nonmagnetic spacer layer 364.

In this case, the combined magnetization of the first spin-polarized component layer 3662 and the second spin-polarized component layer 3666 provides a second tapered magnetization that is free to rotate (e.g., oscillate) about a vertical axis during programming of the MRAM cell 180. In this case, the combined magnetization of the first spin-polarized component layer 3662 and the second spin-polarized component layer 3666 provides an additional taper magnetization (i.e., a second taper magnetization) that is coupled to the first taper magnetization of the spin torque layer 362. During programming, upon application of current through the spin-torque layer 362, the second nonmagnetic spacer layer 364, and the spin-polarizing layer 366, the second tapered magnetization and the first tapered magnetization precess about a vertical axis parallel to the vertical direction of the magnetization of the reference layer 132. The coupling mode between the first tapered magnetization and the second tapered magnetization is preferably any one of the first mode, the second mode, or the third mode shown in fig. 15 to 17.

Referring to fig. 20, a third configuration of an exemplary spin-transfer torque MRAM cell 180 in accordance with embodiments of the present disclosure may be derived from the second configuration of the exemplary spin-transfer torque MRAM cell 180 by exchanging the positions of the first spin-polarized component layer 3662 and the second spin-polarized component layer 3666. In this case, the second spin-polarized component layer 3666 may be in contact with the second nonmagnetic spacer layer 364. The exemplary spin-transfer torque MRAM cell 180 in the third configuration may operate in the same manner as the exemplary spin-transfer torque MRAM cell 180 in the first and second configurations.

Fig. 21 illustrates a fourth configuration of an exemplary spin-transfer torque MRAM cell 180, in accordance with embodiments of the present disclosure. The fourth configuration of the example spin-transfer torque MRAM cell 180 may be derived from the first, second, and third configurations of the example spin-transfer torque MRAM cell 180 by replacing the spin torque layer 362 having the first tapered magnetization with a spin torque layer 462 having an in-plane magnetization (i.e., having a negative uniaxial magnetic anisotropy). In other words, the axial component of the magnetization of the spin-torque layer 462 can be zero, and the magnetization (e.g., magnetization direction) of the spin-torque layer 462 can consist of an in-plane component. In this case, the total magnetization of the spin-torque layer 462 is the same as the in-plane magnetization component of the spin-torque layer 462.

In one embodiment, the spin-torque layer 462 comprises a uniform negative uniaxial magnetic anisotropy material. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. In one embodiment, the spin-torque layer 462 comprises and/or consists essentially of a cobalt-iridium alloy. The material composition of the cobalt iridium alloy may be selected to provide negative uniaxial magnetic anisotropy. In one embodiment, the cobalt-iridium alloy may include cobalt atoms at an atomic concentration in a range of 60% to 98% (such as 70% to 90%), and iridium atoms at a remaining atomic concentration. In the illustrative example, with Co_0.8Ir_0.2K of cobalt iridium alloy₁A value of about-0.6 × 106J/m³In another embodiment, the spin-torque layer 462 comprises and/or consists essentially of a cobalt-iron alloy. The material composition of the cobalt-iron alloy may be selected to provide negative uniaxial magnetic anisotropy. In one embodiment, the cobalt-iron alloy may include cobalt atoms at an atomic concentration in a range of 80% to 99.8% (such as 90% to 99.5%), and iron atoms at a remaining atomic concentration. In the illustrative example, with Co_0.9Ir_0.1K of cobalt-iron alloy of composition₁A value of about-0.99 × 10⁶J/m³. In another embodiment, the spin torque layer 462 includes and/or consists essentially of a cobalt iron boron (CoFeB) alloy. The thickness of the spin-torque layer 462 can be in the range of 1nm to 10nm, such as 1.5nm to 6nm, although lesser and greater thicknesses can also be employed.

In another embodiment, the spin-torque layer 462 includes a multilayer stack comprising multiple repetitions of a first magnetic material layer and a second magnetic material layer. The first magnetic material layer may include and/or may consist essentially of the first magnetic material. The second magnetic material layer may include and/or may consist essentially of a second magnetic material.

In one embodiment, the first magnetic material layer comprises cobalt and the second magnetic material layer comprises iron, in one embodiment, the first magnetic material layer consists essentially of cobalt and the second magnetic material layer consists essentially of iron, each first magnetic material layer may have a thickness in a range of 0.3nm to 1nm and each second magnetic material layer may have a thickness in a range of 0.3nm to 1nm the total number of repetitions within spin-torque layer 462 (i.e., the total number of pairs of first and second magnetic material layers) may be in a range of 2 to 20, such as 4 to 10⁶J/m³. The spin torque layer 462 can be used with any of the spin polarizing layers 366 described above in connection with the first through third embodiments.

Various embodiments of the spin torque layer (362 or 462) and the spin polarizing layer 366 combine to provide the benefit of reducing the current density for parallel-to-antiparallel transitions and the current density for antiparallel-to-parallel transitions by providing an initial non-vertical torque to the magnetization of the free layer 136 during an initial phase when the magnetization of the free layer 136 precesses around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer 132 as current begins to flow through the MRAM cell 180.

Specifically, during the write process with current flowing through the stack of MRAM cells 180, the magnetization of both the spin torque layer and the spin polarized layer oscillate at a high frequency, with a taper angle (the angle between the magnetization and the normal axis of the stack layer interface) between 0 and 90 degrees, as shown in fig. 14-18. The oscillating magnetization of the spin-torque layer may have one or more of the following non-limiting benefits, resulting in a lower switching current of the free layer 136. First, the in-plane component of the spin-torque layer 362 magnetization, which is orthogonal to the initial magnetization of the free layer 136, can generate a large spin torque on the free layer 136 to facilitate its initial precession. Second, the oscillating magnetization of the spin-torque layer 362 can cause the aforementioned torque to rotate, which helps to maximize the assist effect throughout the precessional switching process of the free layer 136. Third, the direct current field generated by the magnetization of the spin-torque layer 362 in the free layer 136 may be primarily an in-plane AC field that is also orthogonal to the initial magnetization direction of the free layer. Thus, this also helps to provide rotational torque to assist free layer 136 switching.

While the foregoing refers to certain preferred embodiments, it is to be understood that the disclosure is not so limited. Various modifications to the disclosed embodiments will be apparent to those skilled in the art, and such modifications are intended to be within the scope of the present disclosure. Embodiments employing specific structures and/or configurations are shown in the present disclosure, it being understood that the present disclosure may be practiced in any other compatible structures and/or configurations that are functionally equivalent, provided that such substitutions are not explicitly prohibited or otherwise considered to be impossible by one of ordinary skill in the art. All publications, patent applications, and patents cited herein are incorporated by reference in their entirety.