RU2601734C1 - Method of measuring magnetic field - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to methods of measuring magnetic field and includes affecting a silicon carbide crystal of hexagonal or rhombic polytype containing spin centers with the main quadruplet spin state, along its crystallographic axis of symmetry with focused laser radiation, low-frequency alternating magnetic field and permanent magnetic field. Herewith luminescence intensity is measured of the spin centers with the main quadruplet spin state at different values of constant magnetic field. Within the luminescence intensity change area read is the first curve of dependence of change of the luminescence intensity on constant magnetic field value and calibrated is the first curve by known value of the magnetic field in the flex point of the first curve. Then placed on the said silicon carbide crystal surface is an investigated sample and the luminescence intensity is measured of the spin centers with the main quadruplet spin state at different values of constant magnetic field. Within the luminescence intensity change area read is the second curve of dependence of change of the luminescence intensity on constant magnetic field value and the value of magnetic field created by the analyzed sample in the focal point of laser radiation is determined by the value of the horizontal shift of the second curve relative to the first curve.

EFFECT: technical result is the increase of accuracy of measurements, as well as elimination of heating of the analyzed object.

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Description

The invention relates to nanotechnology and can be used in the field of measuring local weak magnetic fields in quantum optics, biomedicine, as well as in information technologies based on the quantum properties of spins and single photons.

The detection of weak magnetic fields with high spatial resolution at the level of micro- and nanometers is the most important problem in various fields, ranging from fundamental physics and materials science to data storage and biomedical science. For example, at a distance of 10 nm, the spin of one electron creates a magnetic field of about 1 mT, and the corresponding field of one proton creates several nT. A sensor capable of detecting such magnetic fields with nanometer spatial resolution will find a wide range of applications, from detecting magnetic resonance signals from individual electronic or nuclear spins in complex biological molecules to reading classical or quantum bits of information encoded in electronic or nuclear spin memory. A special role in magnetometry is played by optical methods for measuring magnetic fields.

A known method of measuring the magnetic field using the interaction of resonant light with atomic pairs (see D. Budker and M.V. Romalis. - Optical Magnetometry. - Nature Physics, V. 3, p. 227-234, 2007) and without cryogenic technology . The known method includes filling alkali metal vapors with a measuring cell, supplying circularly polarized light that coincides in energy with one of the optical transitions in the alkali metal, polarizing the electron spins of the alkali metal under the influence of light, applying a resonant radio-frequency field that causes a change in the polarization of electronic spins, detecting the change in the optical transmission of the cell at the time of magnetic resonance in the measured magnetic field, the measurement of the frequency of the magnetic resonance and determined e of the magnetic field at the magnetic resonance frequency in accordance with the Zeeman shift of the spin levels in the measured magnetic field.

The known method of measuring the magnetic field, based on the spin properties of the vapors of alkaline elements, allows you to measure magnetic fields with extremely high sensitivity. However, for effective interaction with resonant light, it requires the use of relatively large volumes of atomic vapors (a measuring cell with atomic vapors), which are determined by the limitations caused by collisions of active atoms and loss of coherence in such collisions. These sizes cannot be less than millimeter values. Therefore, the known method cannot provide a measurement of the distribution of magnetic fields with micron and especially submicron resolution.

After the discovery of the emitting properties of nitrogen-vacancy centers (NV centers) in diamond, which allow optical detection of magnetic resonance in the ground state of these centers at room temperature until the detection of magnetic resonance at single centers (see A. Gruber, A. Drabenstedt, C. Tietz , L. Fleury, J. Wrachtrup, C. Von Borczyskowski. - Scanning Confocal Optical Microscopy and Magnetic Resonance on Single Defect Centers. - Science, V. 276, pp. 2012-2014, 1997; J. Wrachtrup, F. Jelezko. - Processing quantum information in diamond. - Journ. Phys .: Condens. Matter, V. 18, S807, 2006), it became possible to create quantum sensors for measuring magnetic fields with nan dimensional resolution. A separate NV center is a carbon (V) vacancy in the nearest coordination sphere, in which one of the four carbon atoms is replaced by a nitrogen (N) atom. NV centers are characterized by the ground triplet spin state (S = 1), in which nonequilibrium filling of spin levels is optically created. In this case, a change in the filling of spin levels under the influence of resonant microwave radiation with which the spin system of NV centers interacts is detected optically from a change in the luminescence of NV centers. The method of magnetometry with spin NV centers is based on the optical detection of magnetic resonance (ODMR) in an external magnetic field, which must be measured.

A known method of measuring the magnetic field using NV centers in diamond at room temperature (G. Balasubramanian, IY Chan, R. Kolesov, M. Al-Hmoud, J. Tisler, C. Shin, C. Kim, A. Wojcik, PR Hemmer , A. Krueger, T. Hanke, A. Leitenstorfer, R. Bratschitsch, F. Jelezko, J. Wrachtrup. - Nanoscale imaging magnetometry with diamond spins under ambient conditions, Nature, V. 455, pp. 648-651, 2008) . The method includes the following sequence of operations: the laser radiation is focused on the active material - a diamond crystal with NV centers, at the point of measurement of the magnetic field and affect the active material with an external constant magnetic field; supplying microwave radiation with an energy close to the fine structure splitting for NV centers (2.8 GHz), which interacts with the spin system of NV centers; register the photoluminescence of NV centers; change the frequency of microwave radiation and measure the frequencies at which there is a change in the luminescence intensity of the NV centers, due to magnetic resonance at the spin levels of the NV centers; measure the frequency of the magnetic resonance and determine the magnitude of the magnetic field from the frequency of the magnetic resonance in accordance with the Zeeman shift of spin levels in the measured magnetic field.

The disadvantage of this method is the use of diamonds with NV centers as an active material for measuring magnetic fields, the technology for which is extremely expensive and relatively poorly developed. In addition, the optical range in the visible region is used, which does not fit well with silicon-based fiber optics, as well as with the transparency band of biological systems. The method is based on measuring frequency deviations depending on the splitting of the Zeeman levels for a triplet state (S = 1), which is significantly less than for a larger spin, for example, a quadruplet state (S = 3/2). The splitting of the fine structure for NV centers in diamond substantially depends on the temperature and stresses in the diamond crystal, which reduces the accuracy of magnetic field measurements and requires additional efforts to exclude temperature effects. It is also necessary to use microwave radiation, which complicates the design, creates additional noise, and also leads to heating of the object of study.

A known method of measuring the magnetic field (see application US 20100315079, IPC G01N 24/10, G01R 33/00, published December 16, 2010), which involves exposing a diamond to a crystal containing spin centers in the form of NV centers with optical radiation to align electronic spins in magnetic field; applying microwave pulses to the electron spin system in such a way as to cause a precession of electron spins around the direction of the magnetic field to be detected, measuring the frequency of this precession, which is linearly dependent on the magnetic field due to a shift of the spin levels due to the Zeeman effect; and detecting the output optical radiation (photoluminescence) from the electron spin system in diamond after the electron spin system in the solid has been excited by optical and microwave radiation, determining the Zeeman shift and thus determining the measured magnetic field from the output radiation.

The disadvantage of this method is the use of diamonds with NV centers as an active material for measuring magnetic fields, the technology for which is extremely expensive and relatively poorly developed. In addition, the method uses the optical range in the visible region, which is poorly compatible with silicon-based fiber optics, as well as with the transparency band of biological systems. In the known method, it is necessary to use a relatively powerful microwave radiation, which complicates the method, creates additional noise.

A known method of measuring the magnetic field (see application EP 2837930, IPC G01N 21/64, G01N 24/10, G01R 33/24, published 02/18/2015), including the effect of optical radiation on a diamond containing NV centers, for aligning electronic spins in a certain spin state; the action of continuous microwave radiation or pulses of microwave radiation on a diamond containing NV centers, so as to cause transitions between the electronic spin states of the NV centers for specific values of the frequency of microwave radiation, which depend on the measured magnetic field, and the registration of the intensity of the output optical radiation emerging from diamond containing NV centers, so as to determine the Zeeman shift and therefore the magnetic field.

The known method of measuring the magnetic field has the same disadvantages that were discussed above.

Recently, vacancy spin centers in silicon carbide (SiC) have been discovered with properties similar to NV centers in diamond; hereinafter, these centers will also be called spin centers with the ground quadruplet spin state (see N. Kraus, VA Soltamov, F. Fuchs, D Simin, A. Sperlich, PG Baranov, GV Astakhov, V. Dyakonov; Magnetic field and temperature sensing with atomic-scale spin defects in silicon carbide, Scientific Reports, 2014), and a method for measuring the magnetic field that is consistent with this solution according to the largest number of essential features and taken as a prototype. The prototype method is based on optical detection of magnetic resonance (ODMR) at spin centers with the main quadruplet spin state in silicon carbide in an external magnetic field, which must be measured. The spin center with the ground quadruplet spin state is a negatively charged silicon vacancy (V _Si ^- ) with spin S = 3/2, interacting with a neutral carbon vacancy (V _C ⁰ ) located along the hexagonal crystallographic axis (c axis) relative to the silicon vacancy and not having a molecular bond with a silicon vacancy. Optical excitation in the near infrared range (780–850 nm) results in alignment of the spins of such spin centers, which creates a nonequilibrium filling of the spin levels. A change in the filling of spin levels upon irradiation of a crystal by high-frequency (HF) radiation with a magnetic resonance frequency leads to a change in the luminescence intensity of spin centers with the ground quadruplet spin state at the moment of magnetic resonance. The prototype method includes exposing a silicon carbide crystal to a hexagonal or rhombic polytype containing spin centers with the main quadruplet spin state S = 3/2 along its crystallographic axis with symmetry of RF radiation with an energy close to the fine structure splitting for spin centers with the main the quadruplet spin state (20-150 MHz), the magnetic field of the test sample and focused laser radiation (with a wavelength of 780-850 nm), while measuring the intensity of the spin luminescence x centers with the ground quadruplet spin state at different frequencies of the RF radiation, measure the frequencies of the RF radiation in the region of variation in the luminescence intensity due to magnetic resonance at the spin levels of the spin centers with the ground quadruplet spin state; measure the frequency of the magnetic resonance and determine the magnitude of the magnetic field of the test sample by the frequency of the magnetic resonance in accordance with the Zeeman shift of spin levels in the measured magnetic field.

The spin Hamiltonian H, which describes the spin levels of the spin center with the ground quadruplet spin state in magnetic field B, has the form

H = g _e β _e BS _z + D [S _z ² -1 / 3S (S + 1)], J,

where B is a constant magnetic field, T;

Sz is the dimensionless operator of projection of the electron spin onto the direction of the external magnetic field;

g _e = 2,002 - a dimensionless quantity called the g-factor and characterizing the gyromagnetic ratio for the electronic magnetic moment of the used spin center with the ground quadruplet spin state in silicon carbide;

β _e = 9.2740 · 10 ^-24 - Bohr magneton, J / T;

D — fine structure splitting for the spin center with the ground quadrupole spin state in silicon carbide, J (MHz), 1 J = 1.509 · 10 ²⁷ MHz; D = 14 - in silicon carbide polytype 6H-SiC, MHz; D = 35 - in silicon carbide polytype 4H-SiC, MHz.

The prototype method is based on measuring frequency deviations depending on the splitting of the Zeeman levels for the ground quadruplet spin state S = 3/2 of the spin centers in silicon carbide. The fine structure splitting for spin centers with the ground quadruplet spin state in silicon carbide, in contrast to NV centers in diamond, is practically independent of temperature and stresses in a silicon carbide crystal, which increases the accuracy of magnetic field measurements and does not require additional efforts to exclude temperature effects . The axes of the ensemble of spin centers with the ground quadruplet spin state in silicon carbide are oriented along the hexagonal crystallographic axis (c axis), in contrast to the ensemble of NV centers in diamond, in which the axes of the NV centers are oriented along one of the four <111> crystallographic axes, and therefore Using ODMR, in one experiment, only one of the four centers can be measured, while in silicon carbide all centers with the ground quadruplet spin state are already aligned by nature itself along one axis.

A disadvantage of the known method of measuring the magnetic field is the need to use RF radiation, which complicates the implementation of the method, and also creates additional noise and heats the test object, which is placed close to the silicon carbide crystal with spin centers with the main quadruplet spin state and in which the magnetic field is measured. It is also necessary to use additional equipment to create RF radiation and the need to supply RF radiation to the point of measurement of the magnetic field using a loop or coil.

The present invention was the development of such a method of measuring the magnetic field, which would be easier to use and which would exclude the use of RF radiation and, therefore, would exclude heating of the object whose magnetic field is measured.

The problem is solved in that the method of measuring the magnetic field includes exposing the silicon carbide crystal to a hexagonal or rhombic polytype containing spin centers with the main quadruplet spin state along its crystallographic axis with symmetry by focused laser radiation, a variable low-frequency magnetic field, and a constant magnetic field. In this case, the luminescence intensity of the spin centers with the ground quadruplet spin state is measured at different values of the constant magnetic field. In the region of variation of the luminescence intensity, the first curve of the dependence of the change in luminescence intensity on the value of the constant magnetic field is taken, the first curve is graduated from the known value of the magnetic field at the inflection point of the first curve. Then, the test sample is placed on the surface of the aforementioned silicon carbide crystal and the luminescence intensity of spin centers with the ground quadruplet spin state is measured at different values of the constant magnetic field. In the region of variation in the luminescence intensity, a second curve is taken of the dependence of the change in luminescence intensity on the magnitude of the constant magnetic field. The magnitude of the magnetic field generated by the test sample at the focal point of the laser radiation is determined by the horizontal shift of the second curve relative to the first curve.

New in the present method is the exposure of a silicon carbide crystal to a hexagonal or rhombic polytype containing spin centers with a basic quadruplet spin state, an alternating low-frequency magnetic field and a constant magnetic field, as well as the fact that the first curve of the dependence of the change in luminescence intensity on the constant magnetic fields, graduate the first curve by the known value of the magnetic field at the inflection point of the first curve, then place on the surface the mentioned th carbide crystal silicon above the sample, remove the second curve changes depending on the magnitude of the luminescence intensity of the constant magnetic field and the magnetic field produced by the test sample at the focal point of the laser radiation is determined by the magnitude of the horizontal shift of the second curve relative to the first curve.

The luminescence of spin centers with the ground quadruplet spin state can be excited by focused laser radiation using confocal optics or using microscopy based on the suppression of spontaneous emission.

Focused laser radiation, an alternating low-frequency magnetic field and a constant magnetic field can act on a silicon carbide nanocrystal containing at least one spin center with a ground quadruplet spin state.

The present method of measuring the magnetic field with micron and submicron resolution is based on the use of physical processes that lead to a change in the photoluminescence intensity of spin centers with the ground quadruplet spin state, which is manifested in the existence of an inflection point on the curve of the dependence of the change in luminescence intensity of spin centers with the ground quadruplet spin state on constant magnetic field. This inflection point corresponds to the moment of antirecrossing of the energy spin levels of these spin centers in the silicon carbide crystal.

This technical solution is illustrated by drawings, where:

in FIG. Figure 1 shows the structural formula of the spin center with the ground quadruplet spin state S = 3/2 (V _s is the silicon vacancy; V _c is the carbon vacancy);

in FIG. Figure 2 shows a diagram of the energy spin levels in the magnetic field of the spin center with the ground quadruplet spin state S = 3/2;

- точка антипересечения энергетических спиновых уровней; Ms - спиновые проекции S=3/2: M_S=+3/2; M_S=-3/2; M_S=+1/2; M_S=-1/2;

- the point of anti-intersection of energy spin levels; Ms - spin projections S = 3/2: M _S = + 3/2; M _S = -3 / 2; M _S = + 1/2; M _S = -1 / 2;

in FIG. Figure 3 shows the first (1) and second (2) curves of the dependence of the luminescence intensity of spin centers with the ground quadruplet spin state in a silicon carbide crystal of the 6H-SiC polytype on the value of the constant magnetic field; ΔB ₁ - the magnitude of the magnetic field created by the test sample in the form of a fragment of a magnetic film;

in FIG. Figure 4 shows the first (3) and second (4) curves of the dependence of the luminescence intensity of spin centers with the ground quadruplet spin state in a silicon carbide crystal of the 4H-SiC polytype on the value of the constant magnetic field; ΔВ ₂ is the magnitude of the magnetic field created by the test sample in the form of a fragment of a magnetic film.

The present method of measuring the magnetic field includes exposing a silicon carbide crystal to a hexagonal or rhombic polytype containing spin centers with the ground quadruplet spin state (Fig. 1) along its crystallographic axis with symmetry by focused laser radiation (for example, a wavelength of 785 nm), with alternating magnetic a low-frequency field (in the frequency range from tens of Hz to units of kHz) with an amplitude, for example, (0.001-0.01) mT and a constant magnetic field. The luminescence intensity is measured (for example, in the region of 850-950 nm) of spin centers with the main quadruplet spin state at different values of the constant magnetic field and in the region of variation in the luminescence intensity, the first curve of the dependence of the change in luminescence intensity on the constant magnetic field is measured. The first curve is graduated by the known value of the magnetic field B at the inflection point of the first curve, at which the antisection of the spin sublevels M _S = -3 / 2 and M _S = 1/2 occurs for the spin center with the ground quadruplet spin state:

B = D / g _e β _e ,

where B is the constant magnetic field at the inflection point of the first curve, mT;

g _e = 2,0028 - a dimensionless quantity called the g-factor and characterizing the gyromagnetic ratio for the electronic magnetic moment of the spin center in silicon carbide;

β _e = 9.2740 · 10 ^-24 - Bohr magneton, J / T;

D = 14 — fine structure splitting for the spin center with the ground quadruplet spin state in silicon carbide of the 6H-SiC polytype, MHz;

D = 35 — fine structure splitting for the spin center with the ground quadruplet spin state in silicon carbide of the 4H-SiC polytype, MHz.

For silicon carbide polytype 6H-SiC B = 0.5 mT, and for silicon carbide polytype 6H-SiC B = 1.25 mT.

The parameter D is independent of the ambient temperature, which increases the accuracy of measurements. Then the investigated sample is placed on the surface of the silicon carbide crystal, the magnetic field of which must be measured. Again, the luminescence intensity of spin centers with the ground quadruplet spin state is measured at different values of the constant magnetic field. In the region of variation in the luminescence intensity, a second curve is taken of the dependence of the change in luminescence intensity on the magnitude of the constant magnetic field. The value ΔB of the magnetic field generated by the test sample at the focal point of the laser radiation is determined by the horizontal shift of the second curve relative to the first curve.

Example 1. By the method described above, the magnetic field of the sample was measured in the form of a fragment of a magnetic film placed on the plane of a silicon carbide crystal of the 6H-SiC polytype, thus creating a close contact of the sample with a thin layer of spin centers with the ground quadruplet spin state in silicon carbide. Laser light with a wavelength of 795 nm and a spot size in diameter of about 1 μm were focused on working material in the form of silicon carbide, and luminescence of spin centers with a wavelength of 850–900 nm was recorded from this region. The measured magnetic field was recorded by the change in luminescence when the magnetic film fragment was placed on the plane of the silicon carbide crystal of the 6H-SiC polytype and the line shift (curve 2, FIG. 3) with respect to curve 1, FIG. 3 registered in the absence of the measured object (magnetic film). The measured field is 0.022 mT.

Example 2. By the method described above, the magnetic field of the sample was measured in the form of a fragment of a magnetic film placed on the plane of a silicon carbide crystal of the 4H-SiC polytype, thus creating a close contact of the test sample with a thin layer of spin centers with the ground quadruplet spin state in silicon carbide. Laser light with a wavelength of 795 nm and a spot size in diameter of about 1 μm were focused on working material in the form of silicon carbide, and luminescence of spin centers with a wavelength of 850–900 nm was recorded from this region. The measured magnetic field was recorded by the change in luminescence when the magnetic film fragment was placed on the plane of the silicon carbide crystal of the 4H-SiC polytype and the line shift (curve 4, Fig. 4) with respect to curve 3, FIG. 4 registered in the absence of the measured object (magnetic film). The measured field was 0.04 mT.

The present method can be used to obtain a magnetic field gradient during the spatial movement of a spot excited by light in a plane perpendicular to the laser beam.

The present method can also be used to detect alternating magnetic fields in the spatial region of the object under study selected using the optical system, while the time of changes in the magnetic field can reach very small values (less than 10 ns).

One of the options for implementing the present method of measuring the magnetic field is to use a surface layer of a silicon carbide crystal containing a high concentration of spin centers with the main quadruplet spin state to measure the distribution of magnetic fields in the form of magnetic resonance imaging at the nanoscale, with the possibility of using molecular organic or a biological structure deposited on the surface of silicon carbide.

Claims (4)

1. A method of measuring a magnetic field, including exposing a silicon carbide crystal to a hexagonal or rhombic polytype containing spin centers with a fundamental quadruplet spin state along its crystallographic axis with symmetry by focused laser radiation, a low-frequency alternating magnetic field, and a constant magnetic field, while measuring luminescence intensity of spin centers with the ground quadruplet spin state at different values of the constant magnetic field and in the region of Changes in the luminescence intensity take the first curve of the dependence of the luminescence intensity on the constant magnetic field, calibrate the first curve with the known value of the magnetic field at the inflection point of the first curve, then examine the sample on the surface of the said silicon carbide crystal, measure the luminescence intensity of spin centers with the main quadrup spin state at different values of the constant magnetic field, in the region of variation of the lumi the second curve of the dependence of the intensity of the luminescence on the magnitude of the constant magnetic field and determine the magnitude of the magnetic field generated by the test sample at the focal point of the laser radiation, the horizontal shift of the second curve relative to the first curve. 2. The method according to p. 1, characterized in that the luminescence of the spin centers with the ground quadruplet spin state is excited by focused laser radiation using confocal optics. 3. The method according to p. 1, characterized in that the excitation of the luminescence of the spin centers with the main quadruplet spin state by focused laser radiation is carried out using microscopy based on the suppression of spontaneous emission. 4. The method according to p. 1, characterized in that the alternating low-frequency magnetic field, constant magnetic field and focused laser radiation act on a silicon carbide nanocrystal containing at least one spin center with a ground quadruplet spin state S = 3/2.

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