CN111819454B - Method and apparatus for measuring the direction of a magnetic field - Google Patents

Abstract

The invention relates to a method for measuring the direction of the magnetic field of an external magnetic field, the method having the steps of introducing (S1) a sample (2) into an external magnetic field, wherein the sample (2) has a defect which predefines the sample ( 2) corresponding orientation direction; generating (S2) an alternating magnetic field in the region of the sample (2), said alternating magnetic field having a predetermined or predeterminable magnetic field direction; measuring (S3) due to the sample (2) ) spin resonance effect caused by the interaction with the alternating magnetic field; and 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 (2), determining ( S4) The magnetic field direction of the external magnetic field.

Description

Method and device for measuring the direction of a magnetic field

technical field

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

Background technique

Determining the magnetic field plays an important role in sensor technology. For example, in a vehicle or with a portable device, a position for navigation can be determined. High-precision magnetic field sensors are also required for the positioning of metallic or magnetic objects. A possible future application of magnetic fields could be human-machine interfaces. With the help of a magnetic field sensor, the electrical currents formed during brain activity are measured.

The determination of the magnetic field by means of magnetic field pulses is known, for example, from WO 2016/118791 A1. For this, samples with diamonds having nitrogen vacancy centers (NV centers) were used.

"Nanoscale imaging magnetometry with diamond spins under ambient conditions" by Balasubramanian (Nature, vol. 455, pp. 648-651, 2008) provides a study of the center of the NV.

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 miniaturizable and cost-effective sensor devices for determining the direction of a magnetic field.

Contents of the invention

The present invention provides a method for measuring the magnetic field direction of an external magnetic field, and provides an apparatus for measuring the magnetic field direction of an external magnetic field.

Preferred embodiments are also given below.

According to a first aspect, the invention therefore relates to a method for measuring the magnetic field direction of an external magnetic field. A sample is introduced into an external magnetic field, the sample having defects which predetermine a corresponding orientation direction of the sample. An alternating magnetic field with a predetermined or predeterminable 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. 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 magnetic field direction of the external magnetic field is determined.

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

Advantages of the invention

The invention makes it possible, on the basis of investigations of samples with defects, to measure the magnetic field direction of an initially unknown external magnetic field. The orientation of the field direction of the alternating magnetic field relative to the orientation direction of the sample is known and taken into account for determining the field direction of the external magnetic field. A sample can be, for example, a solid with a predetermined crystal structure. During crystal growth, defined defects may appear, wherein the orientation of the defects is typically limited to a few directions predetermined by the crystal structure. These orientation directions may already be known due to the method of fabrication of the samples. According to other embodiments, however, these orientation directions can also be determined in a calibration step described further below.

By using an alternating magnetic field, significantly lower energy consumption can be achieved compared to a static magnetic field generated by a constant current, or compared to magnetic field pulses. The method and the device are therefore also suitable, in particular, for use in mobile applications.

According to a preferred development of the method, the measurement of the spin resonance effect comprises the measurement of optically detected magnetic resonance (ODMR) and/or electrically detected magnetic resonance (EDMR). According to an embodiment, measuring the spin resonance effect may comprise: measuring an ODMR spectrum and/or an EDMR spectrum. The influence of the adjustment variables (ie the direction and value of the applied alternating magnetic field) on the spectrum of the electron spin resonance was determined. The measurement of a spectrum is understood to mean the determination of a measurement variable as a function of a variable, wherein the variable is varied in preferably equal steps. According to certain embodiments, however, only certain points in the spectrum are measured. Therefore, it is not necessarily required to completely determine the spectrum.

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 in which the defects are NV centers. The orientation direction corresponds to at least one of four possible orientations of NV centers in the diamond lattice. The orientation direction refers to that axis derived from the arrangement of nitrogen atoms N and vacancies V.

According to a preferred development of the method, the magnetic field direction of the alternating magnetic field is set perpendicularly 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 resonant 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 diamond samples, these measurements were therefore repeated for the four orientation directions.

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

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 of known magnetic field strength and magnetic field direction. The field direction of the alternating magnetic field is changed relative to the 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 value of the spin resonance effect occurs.

According to a development of the method, one of the orientation directions of the sample is determined orthogonally to the magnetic field direction at which the maximum value of the spin resonance effect occurs.

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

Description of drawings

Attached picture:

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

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

Figure 3 shows a diagram of the ground state and excited energy levels of an NV center;

Figure 4 shows a diagram of the angle between the orientation direction of the NV center, the external magnetic field and the alternating magnetic field;

Figure 5 shows a schematic cross-sectional view of the magnetic field arrangement and sample of the present device;

Fig. 6 shows the schematic oblique view of the magnetic field device and the sample of the device;

Figure 7 shows photodetection magnetic resonance of a single NV center for external magnetic fields with different magnetic field strengths;

Figure 8 shows photodetection magnetic resonance of an ensemble (Ensemble) of NV centers;

Figure 9 shows a diagram of possible angular positions of the alternating magnetic field relative to the four orientation axes of the NV center;

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

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

In all figures, identical or functionally identical elements and devices are provided with the same reference signs.

Detailed ways

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 with a fixedly predetermined crystal structure. Sample 2 has defects which predetermine a corresponding orientation direction. In the simplest case, a single defect may be involved, say a single NV center in diamond sample 2 . However, in order to obtain a stronger signal, an ensemble of such defects is preferably involved. In particular, diamond samples 2 with multiple NV centers may be involved. Depending on the orientation in the crystal lattice, NV centers can have four different orientation directions.

The sample 2 is introduced into the magnetic field arrangement 3 of the apparatus 1 or can be introduced into said magnetic field arrangement 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 relative to the orientation direction of the sample 2 . This orientation can either already be known initially or can be determined by a calibration step described further below. The intensity of the excitation is further dependent on the field strength and the field direction of the external magnetic field which is to be investigated by means of the device 1 .

Excitation of spins (of electrons or nuclei) leads to spin resonance effects, which are detected by measuring device 4 of device 1 . The measuring device 4 outputs a measuring signal, which is evaluated by the evaluating device 5 of the device 1 .

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

An exemplary construction of the device 1 is explained in more detail with reference to the following figures.

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

In FIG. 3 the energy levels of the electrons of the NV centers are plotted. The ground state ³ A forms a spin triplet state in which the zero field splitting (ZFS) between the m_s=0 state and the m_s=+/-1 state has a value of 2.87 GHz in the absence of an external magnetic field . If sample 2 is introduced into an external magnetic field, the two m_s=+/−1 states split due to the Zeeman effect.

The NV center further has an excited triplet state ³ E with corresponding quantum numbers m_s= ⁰ , +/−1. Finally, there exists the metastable singlet state ^1A .

The alternating magnetic field of the magnetic field device 3 in the microwave range produces a state change of the electron spins of the NV centers and thus leads to electron spin resonance ESR. However, the invention is not limited to diamonds with NV centers. For example, SiC samples with SiV centers are also suitable. It is also conceivable to use the nuclear spin resonance of nucleons with a nuclear spin that is not extremely small.

In order to measure an external magnetic field, the device 1 is moved into this external magnetic field. Preferably, initialization is performed in a first step. Thus, electrons are initially in the m_s=0, +/−1 state of the ground state ³ A with substantially the same probability. By exciting with green light, the spins are excited into the corresponding spin states of the excited state ^3E . Finally, the electrons reach the m_s=0 ground state via the metastable state ¹ A. After initialization, essentially all electrons are therefore in the m_s=0 ground state.

In a second step, manipulation of the sample 2 is carried out. To this end, an alternating magnetic field is applied via the magnetic field device 3 . The wavelength or frequency of the alternating magnetic field corresponds essentially to the excitation from the m_s=0 state of the ground state ³ A into the m_s=+/−1 state.

In a third step, the spin resonance effect is detected by means of the measuring device 4 and evaluated by the evaluation device 5 , which determines the 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 to say the preferred direction or orientation direction of the NV center) and the static or quasi-static external magnetic field. Thus, the magnetic interaction H_mag between the magnetic moment μ (which points along one of the orientation directions) and the external magnetic field B is given by the product of these two vector quantities:

H_mag=-μ·B=-|μ|·|B|·cos(θ).

In the case of NV centers, the expression for Zeeman splitting is:

H_mag=γ·|B|·cos(θ),

where γ corresponds to the gyroscopic scale of the NV center, γ≈28.024GHz/T. If the emitted frequency f of the alternating magnetic field corresponds to an additional Zeeman splitting:

omega=2πf=(D±γ·|B|·cos(θ)) (1),

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

Further, the direction of the magnetic field is related to the second angle θ_RF between the alternating magnetic field and the orientation direction. The coupling strength B_RF of the alternating magnetic field is thus given by the nutation frequency ω_nut. This is related to the second angle θ_RF:

ω _nut ＝|1/2·B _RF sinθ _RF | (2)

Equation (2) is suitable for spin 1/2 system. The nutation frequency ω_nut of the spin 1 system is larger by a factor of root 2=1.41.

During resonant microwave excitation, the nutation frequency ω_nut determines the perturbation between the energy levels m_s=0, ±1 in the ground state ³ A. From this it is straightforward to determine: how likely is the radiative fluorescence of the NV center to proceed, that is to say relaxation from state ³ E, m_s = 0 into state ³ A, ms = 0; or whether the fluorescence is reduced by non-radiative relaxation, that is to say From state ³ E, ms=+/−1 relaxes via dark state ¹ A into ground state ³ A, m_s=0. Correspondingly, the amplitude of the optically detected fluorescent 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 propagators. When resonantly excited, the propagator for the alternating magnetic field pulse content is:

The angle φ_p corresponds to the phase of the radiated alternating magnetic field. τ_p is the duration during which the alternating magnetic field is emitted. R_i(φ) is the propagator of the high-frequency pulse that rotates the two-level system around the axis i=x, y, z by an angle φ.

The magnitude of the photodetection 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 arrangement 3 . FIG. 5 shows a schematic cross-sectional view of the sample 2 and the magnetic field arrangement 3 , and FIG. 6 shows a schematic oblique view of the sample 2 and the magnetic field arrangement 3 . Thus, the magnetic field arrangement 3 preferably has three mutually perpendicular conductor coils or waveguides 31 , 32 , 33 . Each of the conductor coils or waveguides takes over the task of generating an alternating magnetic field within the sample 2 along a spatial direction. In this way and in combination with conductor coils or waveguides, an alternating magnetic field in any direction can be generated (AC-Vektormagnet (AC vector magnet)). The magnetic field device 3 is thus designed to generate an alternating magnetic field oriented in any direction. According to other embodiments, however, the magnetic field arrangement 3 can also have only two conductor coils perpendicular to one another and can thus generate an alternating magnetic field that is variable within a plane.

Electron spin resonance can be demonstrated via a change in fluorescence properties, that is to say by optical detection magnetic resonance ODMR. In this way, the ODMR spectrum gives an indication of 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 is illustrated for a unique NV center for an external magnetic field B with different magnetic field strengths. In the absence of an external magnetic field (B=0, see FIG. 7 ), the m_s=+/−1 state degenerates, ie only a single resonance occurs, if the frequency of the alternating magnetic field corresponds to zero field splitting. When a magnetic field is applied, two resonance frequencies ω_1 and ω_2 occur, which are further away from each other for greater magnetic field strengths of the external magnetic field. At the resonance frequency, the amplitude A of the ODMR spectrum has a minimum or a dip, respectively, because the state excited by the resonant alternating magnetic field decays via the metastable state ¹ A and thus causes a reduced amplitude in the fluorescence spectrum value. Thus, if the frequency of the alternating magnetic field corresponds to a magnetic transition of the ground state ³ A, for example to a transition from m_s=0 to m_s=1, the amplitude decreases exactly. The principle of action of ODMR is that photoexcitations from the ground state m_s=+/−1 have a high probability of non-radiative decay into the dark state ¹ A, so that these photoexcitations are not available for the fluorescence process for a longer period of time. Accordingly, in the case of resonant excitation using an alternating magnetic field, the fluorescence intensity decreases.

In Figure 8, the ODMR spectrum for an ensemble of NV centers is illustrated. The four orientation directions of the NV centers result in four pairs of two dimples D1-D1 to D4-D4 each with corresponding pairs of resonant frequencies. The depth of the depressions D1 - D1 to D4 - D4 , that is to say the amplitude A in the ODMR spectrum, depends on the second angle θ_RF between the alternating magnetic field and the corresponding orientation direction.

Four possible orientation directions A1 to A4 are illustrated in FIG. 9 , the first orientation direction A1 running 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 maximal for the z direction because the alternating magnetic field oscillates in the x-y plane. Independently of the third angle φ_RF, the л pulse will in this case reverse the occupancy probability. The second to fourth transition probabilities B2 to B4 each have mutually offset minimum values C2 to C4 , that is to say a characteristic dependence on 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 subsequently the direction of the magnetic field changes within this plane. By determining the minima C2 to C4 the third angle φ_RF and thus the orientation directions A2 to A4 in the ODMR spectrum can be determined. Thereby, the pairs of recesses D1-D1 to D4-D4 or the 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, since the different transition probabilities B1 to B4 can often only be distinguished with difficulty for a fixed third angle φ_RF, a variation of the third angle φ_RF is advantageous.

In general, two-dimensional RF magnets are already suitable as magnetic field means 3 for this purpose. Preferably, however, the alternating magnetic field of the magnetic field arrangement 3 is settable in all three spatial directions. Thereby, the accuracy of direction determination can be improved. Preferably, the method is therefore carried out successively for all orientation directions A1 to A4.

If the alternating magnetic field is oriented orthogonally to one of the four possible orientation directions, the resonance line has a stronger dependence on the alternating magnetic field than the other three resonance lines. The resonance thus exhibits the largest signal amplitude in the spectrum. This means that the corresponding depressions in the ODMR spectrum are extremal (minimum). As a result, absolute directions can be assigned to occurring resonance lines. According to equation (1), the evaluation device 5 can determine the magnitude and direction of the magnetic field. In addition to the magnetic field strength, the field direction of the external magnetic field can thus also be determined exactly.

The invention thus makes it possible to assign 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 is already known. The sample 2 is connected with a fixed orientation to the magnetic field device 3 or is inserted into the magnetic field device 3 . Depending on the currents flowing through conductor coils 31 , 32 , 33 , evaluation device 5 can also always calculate the exact magnetic field direction of the alternating field applied by magnetic field device 3 . From this information, the evaluation device 5 can therefore calculate the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample 2 .

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

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

Now, the field direction of the alternating magnetic field changes with respect to the field direction of the external test magnetic field. For example, the second angle θ_RF and the third angle Φ_RF may vary 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 amplitude of the resonances. In particular, those angle pairs (θ_RF, Φ_RF) for which the amplitude in the ODMR spectrum takes a maximum are determined. This can be done, for example, by a fitting calculation. The angle pairs determined in this way span planes which are perpendicular to the corresponding crystallographic direction or orientation direction. The method can be performed for all four orientation directions. From this, 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 . This makes it possible to select the alternating magnetic field orthogonally to the orientation direction during the measurement operation.

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

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

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

In method step S3 , spin resonance effects are measured which are caused by the interaction of the sample 2 with the alternating magnetic field. 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 on the basis of the measured spin resonance effect. For this, the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation direction of the sample 2 is taken into account. A precise calculation may include the steps set forth above.

Claims (10)

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 optical detection magnetic resonance and/or electrical detection 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 claim 1 or 2, 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. Method according to claim 1 or 2, 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.

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