阅读说明：本技术 用于测量磁场方向的方法和设备 (Method and apparatus for measuring the direction of a magnetic field ) 是由 F.普尔克尔 A.布伦奈斯 R.勒尔韦 于 2019-03-06 设计创作，主要内容包括：本发明涉及一种用于测量外部磁场的磁场方向的方法,该方法具有步骤：将样品(2)引入(S1)到外部磁场中,其中样品(2)具有缺陷,所述缺陷预先规定样品(2)的相对应的取向方向；在样品(2)的区域中产生(S2)交变磁场,所述交变磁场具有预先规定的或者可预先规定的磁场方向；测量(S3)由于样品(2)与交变磁场的相互作用引起的自旋共振效应；和在使用所测量的自旋共振效应和使用交变磁场的磁场方向相对于样品(2)的取向方向的定向的情况下,测定(S4)外部磁场的磁场方向。(The invention relates to a method for measuring the magnetic field direction of an external magnetic field, comprising the following steps: introducing (S1) the sample (2) into an external magnetic field, wherein the sample (2) has a defect which predefines a corresponding orientation direction of the sample (2); generating (S2) an alternating magnetic field in the region of the sample (2), said alternating magnetic field having a predefined or predefinable magnetic field direction; measuring (S3) a spin resonance effect due to interaction of the sample (2) with the alternating magnetic field; and determining (S4) the magnetic field direction of the external magnetic field using the measured spin resonance effect and using the orientation of the magnetic field direction of the alternating magnetic field with respect to the orientation direction of the sample (2).)

1. A method for measuring a magnetic field direction of an external magnetic field, having the steps of:

introducing (S1) a sample (2) into the external magnetic field, wherein the sample (2) has a defect which predefines a corresponding orientation direction of the sample (2);

generating (S2) an alternating magnetic field in the region of the sample (2), the alternating magnetic field having a predefined or predefinable magnetic field direction;

measuring (S3) a spin resonance effect due to the interaction of the sample (2) with the alternating magnetic field; and

determining (S4) the magnetic field direction of the external magnetic field using the measured spin resonance effect and using an orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample (2).

2. The method of claim 1, wherein measuring spin resonance effects comprises measuring photodetection magnetic resonance and/or photodetection magnetic resonance.

3. Method according to claim 1 or 2, wherein the sample (2) is diamond, wherein the defect is a NV centre, and wherein the orientation direction corresponds to at least one of four possible orientations of NV centres in a diamond lattice.

4. Method according to any one of the preceding claims, wherein the magnetic field direction of the alternating magnetic field is set perpendicular to one of the orientation directions and the resonance frequency of the measured spin resonance effect is determined and assigned to the orientation direction, and wherein the magnetic field direction is determined using the resonance frequency assigned to the orientation direction.

5. The method according to claim 4, wherein the setting of the alternating magnetic field and the determination of the resonance frequency are performed successively for all orientation directions.

6. The method according to any one of the preceding claims, wherein the orientation direction of the sample (2) is determined in a calibration step by means of the alternating magnetic field.

7. The method of claim 6, wherein the calibrating step comprises the steps of:

introducing the sample (2) into an external test magnetic field having a known magnetic field strength and magnetic field direction;

changing the magnetic field direction of the alternating magnetic field relative to the magnetic field direction of the external test magnetic field and determining a maximum value of the spin resonance effect; and

determining the orientation direction of the sample (2) using those magnetic field directions of the alternating magnetic field in which a maximum of the spin resonance effect occurs.

8. Method according to claim 7, wherein one of the orientation directions of the sample (2) is determined orthogonally to the following magnetic field directions: in the case of the magnetic field direction, a maximum of the spin resonance effect occurs.

9. A device (1) for measuring a magnetic field direction of an external magnetic field, having:

a sample (2), which sample (2) can be introduced into the external magnetic field, wherein the sample (2) has a defect, which specifies a corresponding orientation direction in advance;

a magnetic field device (3), wherein the magnetic field device (3) is designed to generate an alternating magnetic field in the region of the sample (2), wherein the alternating magnetic field has a predefined or predefinable magnetic field direction;

a measuring device (4), the measuring device (4) being configured to measure a spin resonance effect due to an interaction of the sample (2) with the alternating magnetic field; and

an evaluation device (5), the evaluation device (5) being configured to determine the magnetic field direction of the external magnetic field using the measured spin resonance effect and using an orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample (2).

10. Device (1) according to claim 9, wherein the magnetic field means (3) are configured to vary the magnetic field direction of the emitted alternating magnetic field.

Technical Field

The invention relates to a method for measuring a magnetic field direction of an external magnetic field and a corresponding device.

Background

Determining the magnetic field plays an important role in sensor technology. For example, in a vehicle or with a portable device, the orientation for navigation may be determined. In order to measure a metallic or magnetic object, a high-precision magnetic field sensor is also required. A possible future application of the magnetic field may be a human machine interface. With the aid of the magnetic field sensor, the current formed in the brain activity is measured.

The determination of the magnetic field by means of magnetic field pulses is known, for example, from WO 2016/118791 a 1. For this purpose, a sample with diamond having nitrogen vacancy centres (NV centres) was used.

The "Nanoscale imaging with a two dimensional framework conditions" of Balasubramanian (Nature, Vol. 455, p. 648-651, 2008) provides a study of the NV center.

In addition to accurately determining the magnetic field strength, knowledge of the magnetic field direction is also required for many applications. There is therefore a need for a sensor device which can be miniaturized and which is cost-effective in order to determine the magnetic field direction.

Disclosure of Invention

The invention provides a method for measuring the magnetic field direction of an external magnetic field with the features of claim 1 and an apparatus for measuring the magnetic field direction of an external magnetic field with the features of claim 9.

Preferred embodiments are the subject matter of the respective dependent claims.

According to a first aspect, the invention therefore relates to a method for measuring a magnetic field direction of an external magnetic field. A sample is introduced into an external magnetic field, wherein the sample has a defect which specifies a corresponding orientation direction (orientierung) of the sample in advance. An alternating magnetic field having a predefined or predefinable magnetic field direction is generated in the region of the sample. Spin resonance effects are formed due to the interaction of the sample with the alternating magnetic field, and these spin resonance effects are measured. The magnetic field direction of the external magnetic field is determined using the measured spin resonance effect and using the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample (Ausrichtung).

According to a second aspect, the invention therefore relates to a device for measuring the magnetic field direction of an external magnetic field, having a sample which can be introduced into the external magnetic field. The sample has defects which predetermine the corresponding orientation direction. The device further has a magnetic field device which can generate an alternating magnetic field having a predefined or predefinable magnetic field direction in the region of the sample. The device has a measuring device which measures the spin resonance effect caused by the interaction of the sample with the alternating magnetic field. Finally, the device has an evaluation device which determines the magnetic field direction of the external magnetic field using the measured spin resonance effect and using the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample.

THE ADVANTAGES OF THE PRESENT INVENTION

The invention makes it possible to measure the magnetic field direction of an external magnetic field that is unknown at the outset, as a function of the investigation of a sample having defects. The orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample is known and is taken into account for determining the magnetic field direction of the external magnetic field. The sample may be, for example, a solid having a predetermined crystal structure. During crystal growth, defined defects may occur, the orientation of which is typically limited to a few directions predefined by the crystal structure. These orientation directions may already be known due to the manufacturing method of the sample. However, according to other embodiments, these orientation directions can also be determined in a calibration step described further below.

By using an alternating magnetic field, a significantly lower energy consumption can be achieved than with a static magnetic field generated by a constant current, or than with a magnetic field pulse. The method and the device are thus particularly suitable for use in mobile applications.

According to a preferred development of the method, the measurement of the spin resonance effect comprises the measurement of Optical Detection Magnetic Resonance (ODMR) and/or Electrical Detection Magnetic Resonance (EDMR). According to one embodiment, measuring the spin resonance effect may comprise: the ODMR spectrum and/or the EDMR spectrum are measured. The influence of the manipulated variable (i.e. the direction and magnitude of the applied alternating magnetic field) on the spectrum of the electron spin resonance is determined. The measurement of the spectrum is understood to mean the determination of the measured variable as a function of a variable, wherein the variables are varied in preferably the same step size. However, according to a specific embodiment, only specific points in the spectrum are measured. Thus, a complete determination of the spectrum is not necessarily required.

According to other embodiments, the spin resonance effect can also be a nuclear spin resonance effect.

According to a preferred development of the method, the sample is diamond, wherein the defect is an NV centre. The direction of orientation corresponds to at least one of four possible orientations of NV centres in the diamond lattice. The orientation direction refers to the axis derived from the arrangement of nitrogen atoms N and vacancies V.

According to a preferred development of the method, the field direction of the alternating magnetic field is set (eingstellt) perpendicular to one of the orientation directions. The resonance frequency of the measured spin resonance effect is determined and assigned to the orientation direction. The magnetic field direction is determined using the resonance frequency assigned to this orientation direction.

According to a preferred development of the method, the setting of the alternating magnetic field and the determination of the resonance frequency are carried out successively for all orientation directions. In the case of using a diamond sample, these measurements were therefore repeated for four orientation directions.

According to a preferred development of the method, the direction of orientation of the sample is determined in a calibration step using an alternating magnetic field. Therefore, it is not required to know the orientation direction of the sample initially.

According to a preferred embodiment of the method, the calibration step comprises a plurality of individual steps. In this way, the sample is introduced into an external test magnetic field having a known magnetic field strength and magnetic field direction. The magnetic field direction of the alternating magnetic field is changed relative to the magnetic field direction of the external test magnetic field, and the maximum value of the spin resonance effect is determined. The orientation direction of the sample is determined using those magnetic field directions of the alternating magnetic field in which the maximum of the spin resonance effect occurs.

According to a further development of the method, one of the orientation directions of the sample is determined orthogonally to the following magnetic field directions: in the case of the magnetic field direction, a maximum of the spin resonance effect occurs.

According to a preferred development of the device, the magnetic field device is designed to change the magnetic field direction of the emitted alternating magnetic field.

Drawings

The attached drawings are as follows:

fig. 1 shows a schematic block diagram of an apparatus for measuring a magnetic field direction of an external magnetic field according to an embodiment of the invention;

FIG. 2 shows a schematic illustration of a diamond lattice with NV centers;

FIG. 3 shows a diagram of ground states and excited energy levels of NV centers;

FIG. 4 shows a graphical representation of the orientation direction at the center of NV, the angle between the external magnetic field and the alternating magnetic field;

FIG. 5 shows a schematic cross-sectional view of the magnetic field means and sample of the present apparatus;

FIG. 6 shows a schematic perspective view of the magnetic field means and sample of the present apparatus;

FIG. 7 shows a single NV center optically probed magnetic resonance for external magnetic fields with different magnetic field strengths;

FIG. 8 shows the optical probe magnetic resonance of the Ensemble (Ensemble) at the center of NV;

FIG. 9 shows a graphical representation of possible angular positions of an alternating magnetic field with respect to four axes of orientation of the center of NV;

FIG. 10 shows transition probabilities associated with the angular positions illustrated in FIG. 9; and

fig. 11 shows a flow chart of a method for measuring a magnetic field direction of an external magnetic field according to an embodiment of the invention.

Throughout the drawings, identical or functionally identical elements and devices are provided with the same reference numerals.

Detailed Description

Fig. 1 shows a schematic block diagram of a device 1 for measuring an external magnetic field.

The device 1 has a sample 2, wherein the sample 2 is preferably a solid body with a fixedly predefined crystal structure. Sample 2 had defects that predefine the corresponding orientation direction. In the simplest case, a single defect may be involved, say a single NV centre in diamond sample 2. However, to obtain a stronger signal, an ensemble of such defects is preferably involved. In particular, it may relate to a diamond sample 2 having a plurality of NV centres. Depending on the orientation in the crystal lattice, NV centres can have four different directions of orientation.

The sample 2 is introduced into the magnetic field means 3 of the device 1 or can be introduced into said magnetic field means 3. The magnetic field device 3 is designed to generate a resonant alternating magnetic field, which can excite the spins of the electrons or nuclei of the sample. The intensity of the excitation is related to the orientation of the alternating magnetic field with respect to the orientation direction of the sample 2. This orientation may either be already known from the beginning or may be determined by a calibration step described further below. The strength of the excitation is further related to the magnetic field strength and the magnetic field direction of an external magnetic field as follows: the external magnetic field is to be investigated by means of the device 1.

The excitation of the spins (of electrons or nuclei) leads to a spin resonance effect, which is detected by the measuring device 4 of the apparatus 1. The measuring means 4 output a measuring signal which is evaluated by the evaluation means 5 of the device 1.

The evaluation device 5 is designed to determine the magnetic field direction of the external magnetic field using the measurement signal and taking into account the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample.

An exemplary embodiment of the device 1 is explained in more detail in the following figures.

Thus, sample 2 is preferably a diamond sample with NV centres. As shown in fig. 2, nitrogen atoms N and vacancies V are involved in this case, which occur in the crystal lattice of the diamond. Due to the crystal structure of diamond, the NV centre can point in four different directions, which thus predetermine the protruding orientation direction of sample 2.

The energy levels of electrons in NV centres are plotted in fig. 3. Ground state³A forms spin triplets, wherein the Zero Field Splitting (ZFS) between the m _ s =0 state and the m _ s = +/-1 state has a value of 2.87GHz in the absence of an external magnetic field. If sample 2 is introduced into an external magnetic field, the two m _ s = +/-1 states are split due to the zeeman effect.

The NV center further has an excited triplet state³E, the excited triplet state³E has a corresponding quantum number m _ s =0, +/-1. Finally, a metastable singlet state exists¹A。

The alternating magnetic field of the magnetic field device 3 in the microwave range produces a change in the state of the electron spin at the NV centre and thus leads to an electron spin resonance ESR. However, the present invention is not limited to diamonds having NV centres. For example, SiC samples with SiV centers are also suitable. It is also conceivable to use nuclear spin resonance with nuclei which have not very small nuclear spins.

To measure the external magnetic field, the device 1 is moved into the external magnetic field. Preferably, the initialization is performed in a first step. Thus, the electrons are initially in the ground state with substantially the same probability³M _ s =0, +/-1 state for a. The spins are excited to excited states by excitation with green light³E in the corresponding spin state. Finally, the electrons pass through the metastable state¹A reaches m _ s =0 ground state. After initialization, substantially all electrons are thus in the m _ s =0 ground state.

In a second step, the manipulation of the sample 2 is performed. For this purpose, an alternating magnetic field is applied by the magnetic field device 3. The wavelength or frequency of the alternating magnetic field substantially corresponding to that from the ground state³M _ s =0 state to m _ s = +/-1 state of a.

In a third step, the spin resonance effect is detected by means of the measuring device 4 and evaluated by the evaluation device 5, the evaluation device 5 determining the magnetic field direction of the external magnetic field. In this case, the magnetic field direction of the external magnetic field is related to a first angle θ between the magnetic moment (that is, the preferred direction or orientation direction of the NV centre) and the static or quasi-static external magnetic field. Thus, the magnetic interaction H _ mag between the magnetic moment μ (which points in one of the orientation directions) and the external magnetic field B is given by the product of these two vector quantities:

。

in the case of NV centres, the expression for zeeman split content is:

，

where γ corresponds to the gyro proportion of the NV center, γ ≈ 28.024 GHz/T. If the radiated frequency f of the alternating magnetic field corresponds to an additional zeeman-split:

(1)，

magnetic resonance occurs. Here, D =2.87GHz corresponds to the zero-field splitting (ZFS, see fig. 3) referred to above.

Further, the magnetic field direction is related to a second angle θ _ RF between the alternating magnetic field and the orientation direction. Thus, the coupling strength B _ RF of the alternating magnetic field is given by the nutation frequency ω _ nut. This is related to the second angle θ _ RF:

(2)

equation (2) applies to spin 1/2 systems. The nutation frequency ω _ nut of the spin 1 system is larger by a factor of the root number 2= 1.41.

The nutation frequency omega _ nut is determined in the ground state when the resonant microwave is excited³Scrambling between energy levels m _ s =0, ± 1 in a. Thus directly determining: how much possible the radiative fluorescence of the NV center is, that is, from the state³E, m _ s =0 relaxation to state³A, ms = 0; or whether the fluorescence is reduced by non-radiative relaxation, that is to say from the state³E, ms = +/-1 via dark state¹Relaxation of A to the ground state³A, m _ s = 0. Correspondingly, the amplitude of the optically detected fluorescence signal is related to the magnetic field strength | B _ RF | of the alternating magnetic field and the magnetic field direction or the second angle θ _ RF.

The effect of the alternating magnetic field on the scrambling in the ground state can be calculated by the corresponding propagator. At resonance excitation, the propagator for an alternating magnetic field pulse content is:

(3)

the angle p corresponds to the phase of the emitted alternating magnetic field. τ _ p is the following duration: during said duration, the alternating magnetic field is emitted. R _ i (φ) is a propagator of the high frequency pulse as follows: the propagator of the high frequency pulse rotates the two-order system by an angle phi about an axis i = x, y, z.

The amplitude of the photo detection magnetic resonance is thus related to the second angle θ _ RF. The first angle θ and the second angle θ _ RF are illustrated in fig. 4.

The sample 2 is fixedly arranged in the magnetic field means 3. Fig. 5 shows a schematic cross-sectional view of the sample 2 and the magnetic field device 3, and fig. 6 shows a schematic oblique view of the sample 2 and the magnetic field device 3. The magnetic field device 3 thus preferably has three mutually perpendicular conductor coils or waveguides 31, 32, 33. Each of the conductor coils or waveguides undertakes the task of generating an alternating magnetic field along one spatial direction within the sample 2. Thereby and in combination with the conductor coil or waveguide, an alternating magnetic field (AC-Vektormagnet) of arbitrary direction can be generated. The magnetic field device 3 is thus configured to generate an alternating magnetic field oriented in any direction. However, according to other embodiments, the magnetic field device 3 can also have only two conductor coils perpendicular to one another, and thus an alternating magnetic field that can be varied in one plane can be generated.

The electron spin resonance can be demonstrated via a change in the fluorescence properties, that is to say by means of photodetection magnetic resonance ODMR. Thus, the ODMR spectrum gives an explanation about the magnitude of the field to be measured.

In fig. 7, the ODMR spectrum as a function of the frequency f of the alternating magnetic field for a unique NV centre is illustrated for an external magnetic field B with different magnetic field strength. In the absence of an external magnetic field (B =0, see fig. 7), if the frequency of the alternating magnetic field corresponds to a zero field split, the m _ s = +/-1 state degrades, that is to say only a unique resonance occurs. In the case of an applied magnetic field, two resonance frequencies ω _1 and ω _2 occur, which are further away from each other for a larger magnetic field strength of the external magnetic field. In the case of the resonance frequency, the amplitude a of the ODMR spectrum has a minimum or a Dip (Dip), respectively, since the state excited by the resonant alternating magnetic field is via the metastable state¹A decays and thus causes a reduced amplitude in the fluorescence spectrum. Thus, if the frequency of the alternating magnetic field corresponds to the ground state³A magnetic transition, for example corresponding to a transition from m _ s =0 to m _ s =1, then the amplitude is just reduced. The principle of action of ODMR is that photoexcitation from the ground state m _ s = +/-1 possesses non-radiative decay to the dark state¹A high probability that these light excitations are not available for the fluorescence process for a longer time. Thereby, resonance excitation is performed by the alternating magnetic fieldIn the case of (2), the fluorescence intensity decreases.

In FIG. 8, the ODMR spectrum of an ensemble for NV centers is illustrated. The four orientations of the NV center result in four pairs of two recesses D1-D1-D4-D4 with corresponding pairs of resonant frequencies. The depths of the recesses D1-D1 to D4-D4 (that is, the amplitude A in the ODMR spectrum) are related to the second angle θ _ RF between the alternating magnetic field and the corresponding orientation direction.

Four possible orientation directions a 1-a 4 are illustrated in fig. 9, wherein the first orientation direction a1 runs along the z-axis. The alternating magnetic field illustratively has a third angle φ _ RF with respect to the x-axis and lies within the x-y plane.

In fig. 10, the corresponding dependence of the transition probability P as a function of the third angle Φ _ RF is illustrated. The transition probability P is proportional to the resonance amplitude in the ODMR spectrum. Since the alternating magnetic field runs perpendicular to the first orientation direction a1, the first transition probability B1 has a constant course. The nutation frequency is maximum for the z direction because the alternating magnetic field oscillates in the x-y plane. Pi pulse will in this case reverse the probability of occupation, regardless of the third angle φ _ RF. The second to fourth transition probabilities B2 to B4 have minimum values C2 to C4, respectively, which are shifted from each other, that is, a characteristic correlation with the third angle Φ _ RF.

Preferably, an alternating magnetic field applied by the magnetic field device 3 perpendicular to one of the orientation directions a1 to a4 is thus selected, and the magnetic field direction is then changed within the plane. By determining the minima C2 to C4, the third angle φ _ RF can be determined, and the orientation directions A2 to A4 in the ODMR spectra can be determined therefrom. Thus, the recess pairs D1-D1-D4-D4 or corresponding resonance frequencies can be assigned to the respective orientation directions.

In principle, it is possible to determine the orientation direction by measuring at a single third angle φ _ RF. In this case, the magnetic field of the alternating magnetic field does not necessarily have to be variable. However, the variation of the third angle Φ _ RF is advantageous since it is often only difficult to distinguish the different transition probabilities B1 to B4 for a fixed third angle Φ _ RF.

Generally, for this purpose, two-dimensional RF magnets have been suitable as magnetic field means 3. Preferably, however, the alternating magnetic field of the magnetic field device 3 is settable in all three spatial directions. This can improve the accuracy of the direction determination. Preferably, the method is therefore carried out successively for all orientation directions a1 to a 4.

If the alternating magnetic field is oriented orthogonally to one of the four possible orientation directions, the resonance line has a stronger correlation with the alternating magnetic field than the other three resonance lines. The resonance thus exhibits the largest signal amplitude in the spectrum. That is, the corresponding recess in the ODMR spectrum is extremal (minimum). In this way, an absolute direction can be assigned to the resonance line that occurs. According to equation (1), the evaluation means 5 can determine the value and direction of the magnetic field. In addition to the magnetic field strength, the magnetic field direction of the external magnetic field can therefore also be determined exactly.

The present invention thus enables the assignment of peaks in the ODMR spectrum to absolute spatial axes due to the vector control of the alternating magnetic field, that is to say the settable nature of the magnetic field direction of the alternating magnetic field.

According to one embodiment, the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample 2 is already known. For example, due to the manufacturing method of sample 2, the position of the orientation direction of sample 2 can already be known. The sample 2 is connected to the magnetic field device 3 in a fixed orientation or is inserted into the magnetic field device 3. Depending on the currents flowing through the conductor coils 31, 32, 33, the evaluation device 5 can also always calculate the precise magnetic field direction of the alternating field applied by the magnetic field device 3. From this information, the evaluation device 5 can thus calculate the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample 2.

However, according to other embodiments, the orientation can be determined in an initially performed calibration step. This calibration step may be performed before the device 1 is put into operation for the first time or before each measurement of the external magnetic field.

Thus, the sample is first introduced into an external test magnetic field having a known magnetic field strength and magnetic field direction. The magnetic field strength of the test magnetic field may be, for example, 1 mT. The following four pairs of D1-D1 to D4-D4 of electron spin resonance are thereby prevented from degrading: the four pairs of D1-D1-D4-D4 correspond to four possible orientations of NV centers in diamond.

Now, the magnetic field direction of the alternating magnetic field changes with respect to the magnetic field direction of the external test magnetic field. For example, the second angle θ _ RF and the third angle Φ _ RF may be changed in steps of 10 °. With the aid of the measuring device 4, an ODMR spectrum is generated. The recorded ODMR spectra were analyzed in terms of the magnitude of the resonance. In particular, these angle pairs (θ _ RF, Φ _ RF) are determined as follows: for the angle pair, the amplitude in the ODMR spectrum takes a maximum. This can be done, for example, by fitting calculations (Ausgleichslehnung). The angle pair thus determined lies flat against the following plane: the planes are orthogonal to the corresponding crystal or orientation direction. The method can be performed for all four orientation directions. Thus, all relevant angles are known and four possible orientation directions of the NF centers in the diamond lattice are identified.

Thus, the evaluation device 5 can determine four possible orientation directions of the sample 2. In this way, it is possible to select the alternating magnetic field orthogonally to the direction of orientation during the measurement operation.

In fig. 11, a flow chart of a method for measuring a magnetic field direction of an external magnetic field is illustrated, which method may in particular be performed with the above described device 1. Alternatively, the just described calibration of the device 1 may be performed first.

In method step S1, sample 2 is now introduced into an external magnetic field, sample 2 having defects and a corresponding orientation direction.

In a method step S2, an alternating magnetic field is generated, wherein the magnetic field direction is predefinable and preferably variably settable. The magnetic field direction may preferably vary in at least one plane. Preferably, the magnetic field direction is settable in all three spatial dimensions.

In a method step S3, the spin resonance effect, which is caused by the interaction of the sample 2 with the alternating magnetic field, is measured. In particular, ODMR spectra can be detected for this purpose.

In method step S4, the magnetic field direction of the external magnetic field is determined from the measured spin resonance effect. For this purpose, the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample 2 is considered. The precise calculation may include the steps set forth above.