WO2019174975A1

WO2019174975A1 - Method and apparatus for measuring a magnetic field direction

Info

Publication number: WO2019174975A1
Application number: PCT/EP2019/055479
Authority: WO
Inventors: Fabian Purkl; Andreas Brenneis; Robert Roelver
Original assignee: Robert Bosch Gmbh
Priority date: 2018-03-14
Filing date: 2019-03-06
Publication date: 2019-09-19
Also published as: DE102018203845A1; CN111819454A; CN111819454B

Abstract

The invention relates to a method for measuring a magnetic field direction of an external magnetic field, the method comprising the steps: introducing (Sl) a sample (2) into the external magnetic field, wherein the sample (2) has defects, which specify corresponding orientation directions of the sample (2); generating (S2) an alternating magnetic field having a predetermined or predeterminable magnetic field direction in the region of the sample (2); measuring (S3) spin resonance effects due to interactions of the sample (2) with the alternating magnetic field; and determining (S4) he magnetic field direction of the external magnetic field using the measured spin resonance effects, and orientating the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample (2).

Description

description

title

Method and apparatus for measuring a magnetic field direction

The present invention relates to a method for measuring a

Magnetic field direction of an external magnetic field and a corresponding

Contraption.

State of the art

The determination of magnetic fields plays an important role in sensor technology. For example, the direction can be determined for navigation in vehicles or with portable devices. High precision magnetic field sensors are also needed for locating metallic or magnetic objects. A possible future application of magnetic fields may be human-machine interfaces. Magnetic sensors measure the electrical currents that occur during brain activity.

A determination of magnetic fields by means of magnetic field pulses is known, for example, from WO 2016/118791 A1. For this purpose, a sample with a

Diamonds with nitrogen vacancy centers (NV centers) used.

A study by NV Centers provides Balasubramanian, "A lanoscale imaging magnetometry with diamond spins under ambient conditions", Nature 455, pages 648-651, 2008.

Besides the exact determination of the magnetic field strengths is for many

Applications also require knowledge of the magnetic field direction. There is thus a need for miniaturizable and cost-effective

Sensor devices for determining magnetic field directions.

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

Preferred embodiments are the subject of the respective subclaims.

According to a first aspect, the invention accordingly relates to a method for measuring a magnetic field direction of an external magnetic field. A sample is placed in the external magnetic field, the sample having defects that dictate corresponding orientation directions of the sample. An alternating magnetic field with a predetermined or predefinable magnetic field direction is generated in the region of the sample. Spin resonance effects occur due to interactions of the sample with the alternating magnetic field and are measured. The magnetic field direction of the external magnetic field is under

Using the measured spin resonance effects and an orientation of the magnetic field direction of the alternating magnetic field relative to the

Orientation directions of the sample determined.

According to a second aspect, the invention accordingly relates to a device for measuring a magnetic field direction of an external magnetic field, which has a sample which can be introduced into the external magnetic field. The sample has defects which dictate corresponding orientation directions. The device further comprises a magnetic field device, which a

alternating magnetic field with a predetermined or predefinable

Magnetic field direction in the range of the sample can produce. The device has a measuring device which measures spin resonance effects resulting from interactions of the sample with the alternating magnetic field. Finally, the device has an evaluation device which the magnetic field direction of the external magnetic field using the

measured spin resonance effects and an orientation of the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample determined.

Advantages of the invention The invention makes it possible, based on investigations of a sample with defects, the magnetic field direction of an initially unknown outer

Magnetic field to measure. The orientation of the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample is known and is used to determine the magnetic field direction of the outer

Magnetic field used. For example, the sample may be a solid having a given crystal structure. At the

Crystal growth, certain defects may occur, with the orientations of the defects typically limited to a few directions dictated by the crystal structure. These orientation directions may already be known due to the manufacturing process of the sample. However, according to further embodiments, the orientation directions can also be determined in a calibration step described below.

Through the use of alternating magnetic fields, a significantly lower energy consumption can be achieved compared to static magnetic fields, which are generated by a continuous current, or magnetic field pulses.

As a result, the method and the device are particularly suitable for use in mobile applications.

According to a preferred embodiment of the method, the measurement of spin resonance effects comprises measuring an optically detected magnetic resonance (ODM R) and / or an electrically detected magnetic

Resonance (EDMR). In one embodiment, measuring spin resonance effects may include measuring an ODMR spectrum and / or EDMR spectrum. The influence of a manipulated variable, namely the direction and magnitude of the applied alternating magnetic field, on the spectrum of the electron spin resonance is determined. Measuring a spectrum is understood to be the determination of a measured variable as a function of a variable, the variable being varied in preferably the same steps. However, according to certain embodiments, only certain points in the spectrum are measured. A complete determination of the spectrum is therefore not mandatory. According to further embodiments, the spin resonance effects may also be nuclear spin resonance effects.

According to a preferred development of the method, the sample is a diamond, the defects being NV centers. The orientation directions correspond to at least one of the four possible orientations of the NV centers in the diamond crystal lattice. By the orientation direction is meant that axis which results from the arrangement of the nitrogen atom N and the

Defect V yields.

According to a preferred embodiment of the method, the

Magnetic field direction of the alternating magnetic field set perpendicular to one of the orientation directions. A resonance frequency of the measured spin resonance effect is determined and assigned to this orientation direction. The magnetic field direction is determined using the resonance frequency associated with the orientation direction.

According to a preferred embodiment of the method, the setting of the alternating magnetic field and the determination of the resonance frequency is carried out successively for all orientation directions. at

Using a diamond sample thus repeats the measurements for the four orientation directions.

According to a preferred embodiment of the method, the

Orientation directions of the sample using the magnetic

Alternating field determined in a calibration step. Thus, no initial knowledge of the orientation directions of the sample is required.

According to a preferred embodiment of the method, the calibration step comprises a plurality of individual steps. Thus, the sample is introduced into an external test magnetic field of known magnetic field strength and magnetic field direction. The magnetic field direction of the alternating magnetic field is varied relative to the magnetic field direction of the external test magnetic field and maxima of the spin resonance effects are determined. The orientation directions of the sample are determined using those magnetic field directions of the alternating magnetic field at which maxima of the spin resonance effects occur.

According to one embodiment of the method, one of

Orientation directions of the sample orthogonal to a magnetic field direction determined at which a maximum of the spin resonance effects occurs.

According to a preferred embodiment of the device is the

Magnetic field device designed to the magnetic field direction of

emitted magnetic alternating field to vary.

Brief description of the drawings

Show it:

Figure 1 is 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;

Figure 2 is a schematic illustration of the diamond crystal lattice with

a NV center;

Figure 3 is an illustration of the energy levels of the ground state and of

Suggestions from the NV Center;

FIG. 4 is an illustration of the angles between the orientation direction of the

NV center, the external magnetic field and the alternating magnetic field;

Figure 5 is a schematic cross-sectional view of a sample and a

Magnetic field device of the device;

Figure 6 is a schematic oblique view of the sample and the

Magnetic field device of the device;

Figure 7 optically detected magnetic resonances of a single NV center for external magnetic fields with different magnetic field strengths;

FIG. 8 shows optically detected magnetic resonances of an ensemble of NV centers; Figure 9 is an illustration of possible angular positions of a magnetic

Alternating field relative to the four orientation axes of NV centers;

FIG. 10 shows the transition probability as a function of 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.

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

Description of the embodiments

FIG. 1 shows a schematic block diagram of an apparatus 1 for measuring an external magnetic field.

The device 1 has a sample 2, the sample 2 preferably being a solid having a predetermined crystal structure. The sample 2 has defects, which corresponding

Specify orientation directions. In the simplest case, it may be a single defect, such as a single NV center in a diamond sample 2. To obtain a stronger signal, however, it is preferably an ensemble of such defects. In particular, it may be a

Diamond sample 2 to trade with a variety of NV centers. Depending on the orientation in the crystal lattice, the NV centers can have four different ones

Have orientation directions.

The sample 2 is introduced into or introduced into a magnetic field device 3 of the device 1. The magnetic field device 3 is designed to generate a resonant alternating magnetic field, which is the spin of the

Can excite electrons or nucleons of the sample. The strength of the excitation depends on the orientation of the alternating magnetic field relative to the orientation directions of the sample 2. This alignment may be either already known initially or described below Calibration step are determined. Furthermore, the strength of the excitation depends on a magnetic field strength and a magnetic field direction of an external magnetic field, which is to be investigated by means of the device 1.

The excitations of the spin (the electrons or the nucleons) lead to spin resonance effects, which are detected by a measuring device 4 of the device 1. The measuring device 4 outputs a measurement signal, which is evaluated by an evaluation device 5 of the device 1.

The evaluation device 5 is adapted to the magnetic field direction of the external magnetic field using the measuring signal and under

Considering the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample to determine.

Exemplary embodiments of the device 1 will be 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, these are nitrogen atoms N and defects V, which occur in the crystal lattice of diamond. Due to the crystal structure of diamond, the NV centers can point in four different directions, thus providing excellent orientation directions of sample 2.

FIG. 3 shows the energy levels of the electrons of an NV center. The ground state ³ A forms a spin triplet, where a

Zero field splitting (ZFS) has a value of 2.87 GHz between the m_s = 0 state and the m_s = +/- 1 states in the absence of external magnetic fields. If the 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, a metastable single state exists ¹ A. The magnetic alternating field of the magnetic field device 3 in the microwave range generates a change in state of the electron spins of the NV center and thus leads to electron spin resonance, ESR. However, the invention is not limited to diamond with NV centers. For example, SiC samples with SiV centers are also suitable. It is also conceivable to exploit the nuclear magnetic resonance of nucleons with non-vanishing nuclear spin.

For measuring an external magnetic field, the device 1 is moved into this. Preferably, in a first step, an initialization

carried out. Thus, initially the electrons are in the m_s = 0, +/- 1 states of the ground state ³ A with essentially equal probability. By exciting with green light, the spins are excited into the corresponding spin states of the excited state ³ E. Finally, the electrons enter the m_s = 0 ground state via the metastable state ¹ A. After initialization, therefore, essentially all are

Electrons in m_s = 0 ground state.

In a second step, a 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 essentially corresponds to the excitation from the m_s = 0 state into the m_s = +/- 1

Conditions the ground state ³ A.

In a third step, the spin resonance effects by means of

Measuring device 4 detected and evaluated by the evaluation device 5, which determines the magnetic field direction of the external magnetic field. The

Magnetic field direction of the external magnetic field in this case depends on a first angle Q between the magnetic moment, d. H. the preferred direction or orientation direction of the NV center and the static or quasi-static external magnetic field. So the magnetic interaction H_mag is between the magnetic moment m, which is along one of the

Orientation directions shows, and given the outer magnetic field B by the product of these two vectorial quantities: H_mag = - m · B = - | m | · | B | · oo3 (q)

For the NV Center, this expression for the Zeeman split is:

H_mag = Y- | B | -COS (0), where g corresponds to the gyroscopic ratio of the NV center, g »28.024 GHz / T. Magnetic resonance occurs if the irradiated frequency f of the alternating magnetic field corresponds to this additional Zeeman splitting: omega = 2 p / = (D ± g · | β | · cos (0)) (1)

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

Further, the magnetic field direction depends on a second angle 0_RF between the alternating magnetic field and the orientation direction. So is the coupling strength of an alternating magnetic field ß_RF through the

Nutation frequency a> _nut given. This depends on the second angle 0_RF:

6Jnut | l / 2 · BRF sin 0RF | (2)

Formula (2) applies to Spin V systems. The nutation frequency a> _nut of spin 1 systems is larger by a factor of root (2) = 1.41.

The nutation frequency a> _nut determines the rearrangement between the energy levels m_s = 0, ± 1 in the ground state ³ A with resonant microwave excitation. This determines directly how probable it is that a radiating fluorescence of the NV center takes place, ie a relaxation of the State ³ E, m_s = 0 in the state ³ A, ms = 0, or whether the fluorescence is reduced by non-radiative relaxation, ie relaxation from the state ³ E, ms = +/- 1 via the dark state ¹ A in the ground state ³ A, m_s = 0. Accordingly, the amplitude of the optically detected fluorescence signal depends on the Magnetic field strength | ß_RF | and the magnetic field direction or the second angle 0_RF of the alternating magnetic field.

The influence of the alternating magnetic field on the rearrangement in the

Ground state can be calculated by the corresponding propagators. For resonant excitation, the propagator is for a magnetic

Alternating field pulse:

The angle f_p corresponds to the phase of the radiated magnetic

AC field. t_r is the time duration over which the magnetic

Alternating field is radiated. R_i (f) is the propagator for one

Radio-frequency pulse that rotates a two-level system about the axis i = x, y, z by the angle f.

The amplitude of the optically detected magnetic resonance thus depends on the second angle 0_RF. The first and second angles Q, 0_RF are illustrated in FIG.

The sample 2 is fixedly arranged in the magnetic field device 3. Figure 5 shows a schematic cross-sectional view and Figure 6 is a schematic oblique view of the sample 2 and the magnetic field device 3. Thus, the

Magnetic field device 3 preferably three mutually perpendicular conductor coils or waveguides 31, 32, 33. Each of these conductor coils or waveguides takes over the task of a magnetic alternating field along a

Spatial direction within the sample 2 to produce. As a result, and in combination with the conductor coils or waveguides, any direction of the magnetic alternating current can be generated (AC vector magnet). This is the result

Magnetic field device 3 is formed to generate an oriented in any direction magnetic alternating field. However, according to further embodiments, the magnetic field device 3 may also have only two mutually perpendicular conductor coils and thereby generate a variable within a plane alternating magnetic field. The electron spin resonance may be due to a change in the

Fluorescence property are detected, d. H. by optically detected magnetic resonance, ODMR. Thus, the ODMR spectrum provides information about the amplitude of the field to be measured.

FIG. 7 illustrates the ODMR spectrum for a single NV center as a function of the frequency f of the alternating magnetic field for external magnetic fields B with different magnetic field strengths. In the absence of an external magnetic field (B = 0, see Figure 7), the m_s = +/- 1 states are degenerate, i. H. only a single resonance occurs if the frequency of the alternating magnetic field corresponds to zero-field splitting. When creating a

magnetic field occur two resonant frequencies w_1 and w_2, which for larger magnetic field strengths of the external magnetic field further away from each other. At the resonance frequencies, the amplitude A of the ODM R spectrum has a minimum or a dip, since the states excited by the resonant alternating magnetic field decay beyond the metastable state ¹ A and thus have a reduced fluorescence spectrum

Produce amplitude. The amplitude is thus reduced exactly if the frequency of the alternating magnetic field corresponds to a magnetic transition of the ground state ³ A, for example, the transition from m_s = 0 to m_s = 1. The operating principle of the ODMR is that optical excitations from the ground state m_s = +/- 1 have a high probability that they do not radiantly decay into the dark state ¹ A, whereby they are not available for a longer time to the fluorescence process. This is the result

Fluorescence intensity at resonant excitation with the magnetic

Alternating field reduced.

FIG. 8 illustrates the ODMR spectrum for an ensemble of NV centers.

The four orientation directions of the NV centers lead to four pairs of two dips Dl-Dl to D4-D4 with corresponding pairs of

Resonant frequencies. The depth of the sinks Dl-Dl to D4-D4, d. H. the

Amplitude A in the ODMR spectrum depends on the second angle 0_RF between the alternating magnetic field and the respective orientation direction. FIG. 9 illustrates the four possible orientation directions A1 to A4, wherein a first orientation direction A1 extends along the z-axis. By way of example, the alternating magnetic field has a third angle f_RF relative to the x-axis, and lies within the xy plane.

FIG. 10 illustrates the corresponding dependence of the transition probability P as a function of the third angle f_RF. The

Transition probability P is proportional to the resonance amplitude in the ODMR spectrum. Since the magnetic alternating field is perpendicular to the first orientation direction Al, the first transition probability Bl has a constant course. The nutation frequency is maximal for the z-direction because the alternating magnetic field oscillates in the x-y plane. In this case, a tt pulse will invert the occupation probability independently of the third angle f_RF. The second to fourth

Transition probabilities B2 to B4 have mutually shifted minimums C2 to C4, respectively. H. a characteristic dependence on the third angle f_RF.

Preferably, therefore, the alternating magnetic field applied by the magnetic field device 3 is selected perpendicular to one of the orientation directions A1 to A4, and then the magnetic field direction within the plane is varied. By determining the minimums C2 to C4, the third angles f_RF can be determined and thereby the orientation directions A2 to A4 in the ODMR spectrum can be determined. As a result, the sink pairs Dl-Dl to D4-D4 or the corresponding resonant frequencies of the respective

Orientation directions are assigned.

In principle, it is possible to determine the orientation directions by measuring at a single third angle f_RF. In this case, the

Magnetic field direction of the alternating magnetic field is not necessarily variable. However, since different transition probabilities Bl to B4 are often difficult to distinguish for a fixed third angle f_RF, variation of the third angle f_RF is advantageous. In general, this is already a two-dimensional RF magnet as

Magnetic field device 3 suitable. Preferably, the magnetic

Alternating field of the magnetic field device 3, however, in all three

Room directions adjustable. This can reduce the accuracy of the

Direction determination can be increased. Preferably, the method is thus carried out successively for all orientation directions A1 to A4.

If the alternating magnetic field is aligned orthogonal to one of the four possible orientation directions, the resonance line has a stronger dependence on the alternating magnetic field compared to the other three resonance lines. This resonance thus shows the largest

Signal amplitude in the spectrum. That the corresponding dip in the ODMR spectrum is extremal (minimum). Thus, the resonance lines occurring can be assigned an absolute direction. According to equation (1), the

Evaluation device 5 determine the magnetic field in magnitude and direction.

In addition to the magnetic field strength, the magnetic field direction of the external magnetic field can thus be determined exactly.

The invention thus makes it possible due to the vectorial control of the alternating magnetic field, d. H. the adjustability of the magnetic field direction of the alternating magnetic field, the peaks in the ODMR spectrum absolute space axes assign.

According to one embodiment, the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample 2 is already known. For example, due to the manufacturing process of the sample 2, the position of the orientation directions of the sample 2 may already be known. The sample 2 is connected in fixed orientation with the magnetic field device 3 or inserted into this. On the basis of the currents flowing through the conductor coils 31, 32, 33, the evaluation device 5 can furthermore always calculate the exact magnetic field direction of the alternating field applied by the magnetic field device 3. Based on this information, the

Evaluation device 5 thus calculate the orientation of the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample 2. However, according to further embodiments, this alignment may be determined in an initially performed calibration step. This

Calibration step may be performed before the first start-up of the device 1 or before each measurement of an external magnetic field.

Accordingly, the sample is first introduced into an external test magnetic field with known magnetic field strength and magnetic field direction. The magnetic field strength of the test magnetic field may be 1 mT, for example. This prevents the four pairs of the electron spin resonance Dl-Dl to D4-D4 which correspond to the four possible orientations of the NV center in the diamond from being degenerate.

Now, the magnetic field direction of the alternating magnetic field is varied relative to the magnetic field direction of the external test magnetic field. For example, the second angles 0_RF and third angle f_RF can be varied in steps of 10 °. By means of the measuring device 4 ODMR spectra are generated. The recorded ODMR spectra are analyzed for the amplitude of the resonance. In particular, those angle pairs (0_RF, f_RF) are determined for which the amplitude in the ODMR spectrum assumes the maximum value. This can be done for example by a compensation calculation. The pairs of angles determined in this way span the plane which is orthogonal to the corresponding crystal direction or orientation direction. This method can be performed for all four orientation directions. Thus, all relevant angles are known and the four possible orientation directions of the NF center in the diamond lattice are identified.

The evaluation device 5 can thus determine the four possible orientation directions of the sample 2. This makes it possible, in measuring operation the

alternating magnetic field orthogonal to the orientation directions to choose.

FIG. 11 is a flowchart of a method for measuring a

Magnetic field direction of an external magnetic field illustrated, which can be carried out in particular with a device 1 described above. optional First, a just described calibration of the device 1

be performed.

In a method step S1, a sample 2 is now introduced into the external magnetic field, wherein the sample 2 defects and corresponding

Orientation directions.

In a method step S2, an alternating magnetic field is generated, wherein the magnetic field direction can be predetermined and is preferably variably adjustable. The magnetic field direction can preferably be varied at least in one plane.

Preferably, the magnetic field direction is adjustable in all three spatial dimensions.

In a method step S3, spin resonance effects are measured, which result from interactions of the sample 2 with the alternating magnetic field. In particular, an ODMR spectrum can be detected for this purpose.

In a method step S4, the magnetic field direction of the outer

Magnetic field determined in dependence of the measured spin resonance effects. For this purpose, the orientation of the magnetic field direction of the magnetic

Alternating field relative to the orientation directions of the sample 2 taken into account. The exact calculation may include the steps explained above.

Claims

claims

A method of measuring a magnetic field direction of an outer

Magnetic field, with the steps:

Introducing (Sl) a sample (2) into the external magnetic field, wherein the sample (2) has defects which correspond to

Specify orientation directions of the sample (2);

Generating (S2) an alternating magnetic field with a predetermined or predetermined magnetic field direction in the region of the sample (2);

Measuring (S3) spin resonance effects due to interactions 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 effects and an orientation of the magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample (2).

2. The method of claim 1, wherein measuring spin resonance effects comprises measuring an optically detected magnetic resonance and / or an electrically detected magnetic resonance.

The method of claim 1 or 2, wherein the sample (2) is a diamond, wherein the defects are NV centers, and wherein the

Orientation directions of at least one of the four possible

Alignments of the NV centers in the diamond crystal lattice correspond.

4. The 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 a resonance frequency of the measured spin resonance effect is determined and this

Orientation direction is assigned, and wherein the magnetic field direction using the associated with the orientation direction Resonant frequency is determined.

5. The method of claim 4, wherein adjusting the alternating magnetic field and determining the resonance frequency is performed successively for all orientation directions.

6. The method according to any one of the preceding claims, wherein the

Orientation directions of the sample (2) by means of the magnetic

Alternating field can be determined in a calibration step.

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

Introducing the sample (2) in an external test magnetic field with known magnetic field strength and magnetic field direction;

Varying the magnetic field direction of the alternating magnetic field relative to the magnetic field direction of the outer test magnetic field, and determining maxima of the spin resonance effects; and

Determining the orientation directions of the sample (2) using those magnetic field directions of the alternating magnetic field in which maxima of the spin resonance effects occur.

8. The method of claim 7, wherein one of the orientation directions of the sample (2) is determined orthogonal to a magnetic field direction, wherein a maximum of the spin resonance effects occurs.

9. A device (1) for measuring a magnetic field direction of an external magnetic field, comprising: a sample (2) which can be introduced into the external magnetic field, wherein the sample (2) has defects, which corresponding

Specify orientation directions; a magnetic field device (3) which is adapted to generate a magnetic alternating field with a predetermined or predeterminable magnetic field direction in the region of the sample (2); a measuring device (4), which is designed

To measure spin resonance effects due to interactions of the sample (2) with the alternating magnetic field; and an evaluation device (5), which is designed to, the

Magnetic field direction of the external magnetic field using the measured spin resonance effects and an alignment of the

Magnetic field direction of the alternating magnetic field relative to the orientation directions of the sample (2) to determine.

10. Device (1) according to claim 9, wherein the magnetic field device (3) is adapted to vary the magnetic field direction of the emitted alternating magnetic field.