阅读说明：本技术 用于磁芯的基于颗粒的各向异性复合材料 (Particle-based anisotropic composite material for magnetic cores ) 是由 A·R·M·斯里达 T·布伦施维勒 叶穗莹 L·德尔·卡洛 J·O·阿曼 于 2020-06-04 设计创作，主要内容包括：磁芯包括各向异性复合材料,本身包括基体材料(例如,介电、非磁性材料,优选顺磁性材料)和磁性排列的铁磁颗粒。后者可以例如包括微米和/或纳米级的颗粒。这种颗粒在基体材料内形成颗粒链,其中该链形成磁传导的渗透路径。所述路径沿第一方向延伸,由此各链基本上沿该第一方向延伸,同时沿垂直于第一方向并且可能垂直于垂直于第一方向和第二方向的第三方向的第二方向彼此不同且远离。优选在颗粒之间形成颈缩桥。还公开了相关装置(例如,电感器、放大器、变压器等)和制造方法。(The magnetic core comprises an anisotropic composite material, itself comprising a matrix material (e.g., a dielectric, non-magnetic material, preferably a paramagnetic material) and magnetically aligned ferromagnetic particles. The latter may for example comprise micro-and/or nano-scale particles. Such particles form particle chains within the matrix material, wherein the chains form magnetically conductive percolation paths. The path extends in a first direction, whereby the chains extend substantially in the first direction, while being different and distant from each other in a second direction perpendicular to the first direction and possibly to a third direction perpendicular to the first and second directions. Preferably necking bridges are formed between the particles. Related devices (e.g., inductors, amplifiers, transformers, etc.) and methods of manufacture are also disclosed.)

1. A magnetic core, comprising:

anisotropic composite material comprising at least:

a base material; and

the magnetically aligned ferromagnetic particles form chains of the particles within the matrix material, the chains forming magnetically conductive percolation paths extending along a first direction, whereby the chains each extend along the first direction while being distinct and remote from each other along a second direction perpendicular to the first direction.

2. The magnetic core of claim 1, wherein:

the particles comprise micron-sized particles.

3. The magnetic core of claim 1, wherein:

the particles include a first particle of a first type and a second particle of a second type, the second particle having an average diameter smaller than the first particle; and

the second particles form necking bridges between the first particles in the first direction.

4. The magnetic core of claim 3, wherein:

the first particles comprise micro-scale particles and the second particles comprise nano-scale particles.

5. The magnetic core of claim 3, wherein:

the first and second particles have an average sintering temperature substantially below the melting temperature of the matrix material.

6. The magnetic core of claim 1, wherein:

the chain is arranged substantially according to one of: parallel line configurations, concentric circle ring configurations, and racetrack configurations of particle chains.

7. The magnetic core of claim 1, wherein:

the matrix material is a dielectric, non-magnetic material.

8. The magnetic core of claim 7, wherein:

the matrix material is a paramagnetic material.

9. The magnetic core of claim 1, wherein:

the composite material comprises 10 to 50 volume percent of ferromagnetic particles.

10. The magnetic core of claim 1, wherein:

the chains of particles form magnetically conductive percolation paths extending in a first direction and a third direction, the third direction being perpendicular to the first direction and the second direction.

11. A magnetic device comprising a core, the core comprising:

anisotropic composite material comprising at least:

a base material; and

the magnetically aligned ferromagnetic particles form chains of the particles within the matrix material, the chains forming magnetically conductive percolation paths extending along a first direction, whereby the chains each extend along the first direction while being distinct and remote from each other along a second direction perpendicular to the first direction.

12. The magnetic device of claim 11, wherein:

the device is one of the following devices: inductors, transformers, amplifiers, and power supply devices.

13. A method of manufacturing a magnetic core, the method comprising:

providing a matrix material comprising ferromagnetic particles; and

applying a magnetic field to magnetically align ferromagnetic particles in a matrix material to form an anisotropic composite material for a magnetic core, wherein chains of the particles are formed within the matrix material, the chains forming magnetically conductive percolation paths extending along a first direction, whereby the chains each extend along the first direction while being different from and remote from each other along a second direction perpendicular to the first direction.

14. The method of claim 13, wherein:

the ferromagnetic particles comprise micron-sized particles.

15. The method of claim 14, wherein:

the strength of the applied magnetic field is at least 20 mT.

16. The method of claim 13, wherein:

the ferromagnetic particles in the matrix material comprise 10 to 50 volume percent of the formed composite material.

17. The method of claim 13, wherein:

the ferromagnetic particles comprise a first particle of a first type and a second particle of a second type, the second particle having an average diameter smaller than the first particle, an

The method further comprises forming necking bridges between the first particles while applying said magnetic field, so as to bridge the latter in the first direction due to the second particles.

18. The method of claim 17, wherein:

forming the necked bridge includes sintering the first particle and the second particle.

19. The method of claim 13, wherein:

the method further includes immobilizing the chains in the formed anisotropic composite.

20. The method of claim 17, wherein:

the first particles comprise micro-scale particles and the second particles comprise nano-scale particles.

21. The method of claim 13, wherein:

applying the magnetic field to arrange the chains according to one of: parallel line configurations, concentric circle ring configurations, and racetrack configurations of particle chains.

22. The method of claim 13, wherein:

the magnetic field is applied using one or each of a permanent magnet and an electromagnet.

23. The method of claim 13, wherein:

the anisotropic composite material is formed by repeating the steps of claim 13 to successively form layers of the composite material.

24. The method of claim 13, wherein:

a matrix material is disposed in the structured template to constrain the shape of the finally formed composite.

25. The method of claim 13, wherein:

the method further comprises integrating the obtained magnetic core in the device.

Background

The present disclosure relates generally to the field of magnetic cores, such as those used in inductors, amplifiers, transformers, and power supply devices, and methods of making such devices and such magnetic cores. And more particularly to magnetic cores obtained by aligning ferromagnetic particles into chains by applying a magnetic field.

The magnetic core is an object made of a magnetic material having a high magnetic permeability. Such materials are used to confine and guide magnetic fields in various devices. Typically, magnetic cores are used to significantly increase the magnetic field strength in electromagnetic coils. Nevertheless, side effects are observed, for example in applications such as transformers and inductors, which are mainly due to eddy currents (in ac installations). This results in frequency dependent energy losses. Different manufacturing methods have been proposed which rely in particular on laminating thin film magnetic cores, radial magnetic field directed sputtering of thin film magnetic cores or uniformly distributed particles (composite materials forming inductor cores).

Disclosure of Invention

According to a first aspect, the invention is embodied as a magnetic core. The core comprises an anisotropic composite material, itself comprising a matrix material (e.g., a dielectric, non-magnetic material, or paramagnetic material) and magnetically aligned ferromagnetic particles. The particles may for example comprise micro-and/or nano-particles. They form particle chains in the matrix material. Such chains form a percolation path for magnetic conduction. The path extends in a first direction, whereby the chains extend substantially in the first direction. However, the chains remain distinct and distant from each other along a second direction perpendicular to the first direction and possibly along a third direction perpendicular to both the first and second directions. The composite material preferably comprises 10 to 50 volume percent of ferromagnetic particles. The chains may be arranged, for example, substantially according to a parallel line configuration, a concentric circular ring configuration, or a racetrack configuration of particle chains.

This solution relies on a magnetic assembly of ferromagnetic particles, where the penetration of the particles is applied in the direction of the applied magnetic field during manufacture, while being suppressed (or mitigated) in the perpendicular direction. This results in an increase in effective permeability in the direction of the applied magnetic field while suppressing (or at least mitigating) electrical conduction, thereby suppressing eddy currents in the perpendicular direction. I.e. the magnetic flux increases in the direction of the chain, while the electrical conduction and eddy currents decrease in the perpendicular direction. As one may appreciate, this approach advantageously allows for a fast and low cost manufacturing process as compared to, for example, thin film microfabrication.

Preferably, the particles comprise a first particle (first type) and a second particle (second type), wherein the second particle has a smaller average diameter than the first particle. For example, the first particles may comprise micron-sized particles, and the second particles may comprise nano-sized particles. The second particles may advantageously be used to form necking bridges between the first particles, bridging the latter in the first direction.

The first and second particles will preferably have an average sintering temperature significantly below the melting temperature of the matrix material to allow for a sintering process to enhance the mechanical stability and permeance of the chains.

According to another, but related, aspect, the invention is embodied in a magnetic device that includes a magnetic core as described above. The device may be, in particular, an inductor, a transformer, an amplifier or a power supply device.

According to another aspect, the invention is embodied in a method of manufacturing a magnetic core as described above. According to the method: a matrix material is provided comprising ferromagnetic particles, and a magnetic field is applied to magnetically align the ferromagnetic particles in the matrix material to form an anisotropic composite material for a magnetic core. Consistent with the first aspect of the invention mentioned above, this is performed to form chains of particles (e.g. comprising particles of micron-scale) within the matrix material, wherein the chains form magnetically conductive percolation paths extending in the first direction. That is, the chains each extend along a first direction while being different from and distant from each other along a second direction perpendicular to the first direction, as described above. As also noted above, the ferromagnetic particles incorporated in the matrix material may typically comprise 10 to 50 volume percent of the formed composite.

The applied magnetic field preferably has a strength of at least 20 mT. The magnetic field may in particular be applied using permanent magnets and/or electromagnets. In all cases, the magnetic field may be applied such that the chains are arranged according to a parallel line configuration, a concentric circular ring configuration, or a racetrack configuration of particle chains.

Preferably, the introduced particles comprise first particles (i.e., particles of a first type), and second particles (of a second type), wherein the average diameter of the second particles is less than the average diameter of the first particles.

For example, the first particles may comprise micron-sized particles and the second particles may comprise nano-sized particles, as previously described. The method may then include forming necking bridges between the first particles while applying the magnetic field to bridge the first particles due to the second particles and along the first direction. The formation of the necking bridge may also rely on a sintering process to sinter the first and second particles.

Finally, if desired, the chains are fixed in the composite material formed, for example by cross-linking a matrix material, which may be a photopolymer or a heat-curable epoxy resin, for example. Note that the composite material may be formed by successively forming layers of the composite material, which may be achieved by repeating the above-described method steps.

In a preferred embodiment, a matrix material is provided in the structured template to limit the shape of the finally formed composite.

For the sake of completeness, the obtained magnetic core may finally be integrated in one device to obtain, for example, inductors, amplifiers or transformers, power supply devices, etc.

An apparatus and method embodying the invention will now be described by way of non-limiting example and with reference to the accompanying drawings.

Drawings

The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present disclosure, in which:

FIG. 1 is a top view illustrating a chain of particles formed in a magnetic core according to an embodiment;

FIGS. 2A and 2B show necking bridges formed between particles of the chain shown in FIG. 1, referred to in the examples;

FIG. 3 is a photomicrograph of an actual core, including the linear chain of FIG. 1, according to an embodiment;

fig. 4 is a top view of a variation of the linear configuration of the device of fig. 1. Fig. 4 shows a magnetic core exhibiting concentric chains in accordance with an embodiment.

FIG. 5 is a 3-dimensional (3D) view of another variation of a magnetic core according to an embodiment, wherein the strands relate to percolation paths in two perpendicular directions but not between parallel strand rows;

fig. 6 is a 3D view of a toroidal inductor including a magnetic core according to an embodiment;

FIG. 7 is a graph representing a possible multimodal distribution of ferromagnetic grain sizes in a magnetic core according to an embodiment; and

FIG. 8 is a flow diagram illustrating high-level steps of a magnetic core manufacturing method according to an embodiment.

The drawings shown in fig. 1, 2A, 2B and 4-6 show simplified representations of devices or parts thereof as are involved in the embodiments. In particular, the particle chains depicted in fig. 1, 2A, 2B, 4 and 5 are intentionally schematically represented. The features depicted in the drawings are not necessarily drawn to scale. In the figures, elements that are similar or functionally similar have been assigned the same reference numerals unless otherwise stated.

Detailed Description

Referring to fig. 1-5, one aspect of the invention is first described, relating to a magnetic core 10, 10a, 10 b. Note that the reference numerals 10a, 10b refer to variations of the core 10. Although the features of the present magnetic core are described primarily with reference to core 10 of fig. 1 or 2A-2B, it will be apparent to those skilled in the art that cores 10a, 10B may include similar features.

Basically, the core 10 comprises an anisotropic composite material comprising a matrix material 30 and magnetically aligned ferromagnetic particles 11, 12.

As seen in fig. 1-3, such particles 11, 12 form particle chains 20 in a matrix material 30. Such chains 20 form a percolation path for magnetic conduction. The conductive path extends in a first direction (parallel to the x-axis in fig. 1-2A, 2B). That is, each chain 20 forms a conductive path and thus extends in the first direction. In contrast, along a second direction perpendicular to the first direction (parallel to the z-axis in fig. 1-2A, 2B), the chains are different and therefore far from each other.

In other words, the magnetically aligned particles 11, 12 form different chains 20, ensuring a percolation path extending along the first direction. Such chains extend along the direction of the magnetic field applied during manufacture, as discussed later with reference to another aspect of the invention. As a result, the chains are substantially parallel to this first direction, while being different from each other and thus distant from each other in a vertical direction.

The magnetic alignment of the particles 11, 12 is converted into a recognizable pattern of aligned particles. I.e. a clear alignment direction and different chains can still be observed, although not perfectly aligned, see fig. 2A, 2B. And as further seen in fig. 2A, 2B, each chain 20 may actually involve links consisting of small 3D clusters of particles 11, 12, as opposed to the "ideal" depiction of fig. 1, 2B, 4 and 5, which indicates links formed by individual particles 11. Thus, the term "chain" of particles as used herein should be interpreted broadly, e.g. as a thread of filaments or particles 11, 12, i.e. formed of ferromagnetic particles 11, 12 as a longitudinal composite structure (e.g. having a locally twisted cylindrical shell). The particle chain may for example comprise a plurality of particles in a direction perpendicular to the percolation path, as shown in fig. 3, while being kept away from each other.

The first direction may be a straight line parallel to a given axis (e.g., the x-axis), resulting in a configuration of parallel lines, as assumed in fig. 1-2A, 2B. In other cases, the first direction may also be curved (due to the magnetic field applied during manufacturing), resulting in the chains still remaining parallel, as assumed in fig. 4. Note that at the height of the radial portion P (shown in fig. 4) of the core 10a, the chains can be considered to extend parallel to the same, approximately linear direction. In general, the applied magnetic field may be designed to align the chains 20 substantially according to a desired pattern, such as a concentric circular ring configuration (as shown in FIG. 4), a racetrack configuration, or a parallel line configuration (as shown in FIG. 1 or 5).

The present solution relies on magnetic assembly of the particles 11, 12 in the matrix material 30. Penetration of the particles is applied in the direction of the applied magnetic field during manufacture, while being inhibited (or at least substantially mitigated) in the perpendicular direction. As one may appreciate, this increases the effective permeability in the direction of the applied magnetic field, while suppressing (or at least mitigating) eddy currents in the perpendicular direction. That is, the magnetic flux is enhanced in the direction of the chain, while the eddy current is reduced in the perpendicular direction, and thus the importance of anisotropic penetration. Note that the percolation path means that there is sufficient mechanical contact between the ferromagnetic parts of the particles 11, 12 to allow magnetic conduction (i.e. the reluctance at the contact point is low).

Interestingly, the magnetically conductive percolation path may extend along two perpendicular directions, as shown in the core 10b of fig. 5, i.e. perpendicular to the first and second directions along the first direction x and the third direction (parallel to the axis y in fig. 5). Thus, the particle chains form parallel chain rows. According to tests conducted by the inventors, it was found that this configuration provides the best results in terms of effective permeability. The magnetic core 10b as shown in fig. 5 can be manufactured layer by layer (in the y-direction) as discussed later. Again, it should be remembered that the description of fig. 5 is schematic; in practice, the real chain looks much like the chain in fig. 3.

In all cases, the present method allows for a fast and low cost manufacturing process, especially when compared to thin film microfabrication processes. The resulting magnetic core 10, 10a, 10b may be used in a variety of applications, such as electromagnets, transformers, amplifiers, power supply devices, motors, generators, inductors, tape heads, and other magnetic components.

All of these will now be described in detail with reference to specific embodiments of the invention. First, the ferromagnetic particles 11, 12 preferably comprise micron-sized particles. Micro (e.g., spherical and/or rod-shaped) particles can be used at various loading concentrations. As observed by the inventors, the composite material obtained on the basis of such particles provides a stable permeability over a wide frequency range.

Micron-sized particles are particles whose characteristic size (for example, their average diameter for spherical particles or their average cross-sectional diameter for rod-shaped particles) is in the micron length range, i.e. between 1 μm and 100 μm. In a variant of or in addition to the micro-sized particles, particles in the sub-micron range may be used, for example, particles in the nano-scale range (having a characteristic dimension between 1 nm and 100 nm). However, it is preferred to rely primarily on microparticles. However, for reasons explained below, it is preferred to use additional, for example nanoscale particles 12 in addition to the micrometer-sized particles 11.

For example, referring to fig. 2A, 2B, the particles 11, 12 may comprise first particles 11 (i.e., particles of a first type) and second particles 12 (of a second type), wherein the second particles 12 have a smaller average diameter than the first particles 11. As seen in fig. 2A, the second particles 12 may form necking bridges 15 between the larger particles 11, bridging the particles 11 in a first direction (see also fig. 2B). As previously mentioned, the necked-down bridges 15 between the first particles (e.g., microparticles) are preferably achieved by incorporating nano-sized ferromagnetic particles into the composite material 30. The sizes of the first particles 11 and the second particles 12 are typically distributed according to a bimodal or multimodal distribution, as shown in fig. 7, where D (p) represents the distribution of the average diameters p of the particles. Of course, different distributions may be considered, for example extending to larger particle sizes.

Accordingly, the second particles 12 may form necks 15 around the contact area of the first particles 11, which gives additional mechanical stability to the entire permeation path. Furthermore, as shown in fig. 2A, the second particles 12 may form additional magnetically permeable permeation pathways. This enhances permeability by providing a lower reluctance magnetic field path at the contact point of the particles. Necking can be achieved, for example, automatically upon application of a magnetic field due to the fringing and contracting fields caused by the particle assembly upon application of the magnetic field.

Note that the necking bridges 15 are also anisotropic-they are applied in the direction of the magnetic field (along the path) and suppressed in the perpendicular direction (no necks are formed between the particles of different chains). The distance between the chains will typically be greater (or on the same order of magnitude) than the diameter of the largest particle (e.g., micron-sized particle). The distance depends on the volume fraction of the particles.

Although a necked bridge may already be obtained due to a suitably applied magnetic field, the sintering process may additionally be considered to improve mechanical stability and flux guiding. For this reason, the particles 11, 12 will preferably have an average sintering temperature that is significantly lower than the melting temperature of the host material 30. For example, permalloy (nickel iron) particles may be used, which have a melting temperature of about 1450 degrees celsius, but in the case of nanoparticles can be sintered at 200 degrees celsius. However, such a sintering process is optional, as the particles may be fixed in the matrix material and thus remain in contact, for example by curing the matrix material.

In this regard, the host (matrix) material 30 is preferably a dielectric, non-magnetic or paramagnetic material. Diamagnetic materials should preferably be avoided, while ferromagnetic materials are in principle excluded. For example, the matrix material 30 may be composed of or include an epoxy material. It may in particular be an epoxy-based negative photoresist, such as SU-8 polymer or a thermally curable epoxy. Silicone and other adhesive materials may also be used. More generally, other materials such as photoresist materials and photosensitive binder photopolymers are contemplated.

It is contemplated that various types of ferroelectric materials may be used to produce the particles 11, 12, starting from materials containing transition metal elements (Fe, Co, Ni), such as transition metal-metalloid alloys, and rare earth magnets. Suitable particles 11, 12 are commercially available as micro-or nanoparticles. The microparticles are preferably produced by a thermal spray process and the nanoparticles are preferably produced by a plasma spray or liquid precipitation process.

In practice, the magnetic core 10 may comprise, for example, between 10 and 50 volume percent (vol%) of ferromagnetic particles 11, 12. This percentage reflects the composition of the core 10, including the matrix material 30 and the first type of ferromagnetic particles 11, and if desired, the second type of particles 12. It corresponds to the volume fraction of all ferromagnetic particles multiplied by 100. That is, a fraction of 50 volume percent refers to 50 volume units of a mixture of either or both types of ferromagnetic particles 11, 12 with sufficient matrix material 30 (and additional material, if desired) to make up the final volume of 100 units. Although slightly larger volume fractions may be considered, the volume fraction of the ferromagnetic particles 11, 12 generally needs to be limited to prevent penetration in undesired directions. Preferably, the volume fraction of the ferromagnetic particles 11, 12 is between 30 and 45 volume percent. For example, in FIGS. 2A-2B, a volume fraction of 38 volume percent is assumed.

If desired, the thickness of the core 10 may be less than one (or a few) millimeters, and may be as small as 100 microns, for example, when molded in a structured mold. The sample may also be thinned mechanically and/or chemically if necessary. The lateral dimensions (length and width) of the sample 10 will typically be larger, for example in the millimeter to centimeter range, depending on the application sought.

Referring more particularly to fig. 6, a further aspect of the invention will now be described, relating to a magnetic device 1 or apparatus comprising a magnetic core 10, 10a, 10b as described above. That is, the core comprises an anisotropic composite material comprising a matrix material 30 and magnetically aligned ferromagnetic particles 11, 12 assembled into chains 20, the chains 20 forming percolation paths for magnetic conduction, as previously described. In embodiments, such devices may be implemented in particular as inductors, transformers, amplifiers or power supply devices, i.e. various applications may be envisaged, including servers, micro-servers, power supplies in data centers, e.g. for cloud computing, and integrated voltage regulators, e.g. for internet of things applications. Such magnetic devices may also be implemented as miniaturized transformers for isolation in various information technology, automotive and aerospace applications, or miniaturized inductors for resonant circuits, for various applications, such as transceivers.

Fig. 6 shows an example of an implementation of such a device 1 as a toroidal inductor based on Printed Circuit Board (PCB) technology, for example for use in an integrated voltage regulator. The inductor utilizes free space inside the PCB to carry the magnetic core 10a as shown in fig. 4. The windings typically involve PCB metal traces and vias. The magnetic core 10a is embedded in a cavity formed inside the PCB. Manufacturing limitations typically impose an upper limit on the core thickness, e.g., a few millimeters. It is well known that both sides of a PCB may be used for mounting electronic components or one side may be used for cooling purposes.

Referring now to fig. 8, a final aspect of the invention is described, relating to a method of manufacturing the magnetic core 10, 10a, 10b, as previously described with reference to fig. 1-5. Aspects of the method have been described implicitly; they will only be briefly described below.

In one embodiment, the method relies on the base material 30 being provided at step S10 in the flowchart of fig. 8. The material 30 may already comprise ferromagnetic particles 11, 12. In a variant, the ferromagnetic particles 11, 12 may be introduced into the matrix material 30, for example after filling the matrix material into the cavity, as assumed in the flow chart step S20 of fig. 8. A magnetic field S30 is then applied to magnetically align the ferromagnetic particles 11, 12 in the matrix material 30. This is done, as previously described, in order to obtain an anisotropic composite material for a magnetic core 10 in which chains 20 of ferromagnetic particles 11, 12 are formed within a matrix material 30. The chains 20 form magnetically conductive percolation paths extending in a first direction, whereby the chains 20 each extend in the first direction while being different from and remote from each other in a second direction perpendicular to the first direction.

The magnetic field S32 is typically applied using a permanent magnet and/or one or more electromagnets. For example, permanent (dipole) magnets may be arranged on each side of the sample holder, with matrix material placed on each side of the sample holder, before or after introduction of the particles. For example, the magnetic field of a cylindrical dipole magnet with a near constant field gap in its center may be used to align the particles 11, 12. In a variant, a similar field configuration may be obtained from stacked current loops. Other variants rely on electromagnets (e.g., horseshoe magnets), which may in fact be used in addition to or as a variant of permanent magnets. In all cases, the magnetic field lines should preferably be aligned with the gravitational field direction to avoid magnetic field disturbances deforming the chain 20. Another less preferred possibility is to rotate the system consisting of sample, sample holder and magnet in the gravitational field during chain formation and fixation. Thus, the gravitational field may be processed during manufacturing to minimize or otherwise take advantage of its effect on the chain 20 to obtain a desired chain design. However, in principle, the effect of gravity has little effect on the chain geometry.

As mentioned above, the particles 11, 12 introduced at step S20 preferably comprise microparticles 11, possibly supplemented with nanoparticles 12. The ferromagnetic particles 11, 12 introduced into the matrix material 30 will preferably represent 10 to 50 volume percent of the finally obtained composite material. Typically, the applied magnetic field requires a strength of at least 20mT to obtain the desired chains 20.

Assuming that the particles 11, 12 comprise ferromagnetic particles 12 having a smaller average diameter than the first particles 11, the method may further comprise forming S34 necking bridges 15 between the first particles 11. The bridge is formed while applying the S32 magnetic field so as to bridge the first particles due to the second particles 12 and along the first direction. As previously described, the chain 20 remains unconnected (or unconnected) in the vertical direction.

Due to the unique magnetic field S32 applied during manufacturing, necking can be achieved, which results in horizontal edges of the contact area and/or a shrinking field. Necking is anisotropic in that it is applied in the direction of the applied magnetic field, while it is suppressed (or at least mitigated) in the perpendicular direction. Also as previously described, a sintering process S34 is preferably included to strengthen the conductive path. That is, the necking bridges 15 may be completed by sintering S34 the first and second particles 11, 12, by applying heat, thereby compacting the particles and forming chains 20 that penetrate as far as possible in the first direction. Sintering is carried out in the presence of the magnetic field. The particles 11, 12 are typically sintered S34 at a temperature substantially below the melting temperature of the matrix material 30.

Finally, the chains 20 may need to be immobilized S36 in the obtained anisotropic composite material. This is typically achieved by curing the matrix material (e.g. by cross-linking a photopolymer or a thermally cured epoxy) while the magnetic field is still present. More generally, the fixation may be achieved by heating the matrix material while maintaining the magnetic field.

In an embodiment, a matrix material is provided in the structured template (e.g. formed on the surface of the device 1) to limit the shape of the finally formed composite material. For example, one or more micro-machined grooves, cavities, etc. may be used to constrain the final shape of the composite material 11, 12, 30 as a whole in a particular direction. As mentioned above, thicknesses as low as 100 microns can be achieved, while the lateral dimensions can be much larger (in practice up to millimetres up to centimetres).

If desired, the anisotropic composite material is formed as a layer-by-layer process, i.e., by continuously forming layers of the composite material, which can be readily accomplished, for example, by repeating steps S10-S36 of S50, as described above. The core 10b as shown in fig. 5 may be produced, for example, using such a layer-by-layer process. That is, it is possible to obtain chains of particles 20 extending along the first direction x but distant from each other in a second perpendicular direction z and forming magnetically conductive percolation paths extending along the first direction x and the third direction y, the latter being perpendicular to the first direction x and the second direction z, to improve the performance.

Finally, the core material obtained at S36 may be packaged for transport or integration at S60 in a specific use device 1 such as described above, for example using common manufacturing techniques for such devices.

While the invention has been described with reference to a limited number of embodiments, modifications, and drawings, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In particular, features (like devices or methods) shown in a given embodiment, variation, or figure can be combined with or substituted for another feature in another embodiment, variation, or figure without departing from the scope of the invention. Various combinations of features described in relation to any of the above embodiments or variations may be envisaged accordingly and still fall within the scope of the appended claims. In addition, many minor modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, many other variations than those explicitly mentioned above may be envisaged. For example, other materials than those explicitly mentioned may be envisaged as long as they exhibit the desired magnetic properties. Additional materials may be used to tailor the chemical (e.g., crosslinking characteristics of the polymer chains) or mechanical (e.g., rheology, viscosity, etc.) characteristics of the composite material, if desired.