Diamond spin sensor

Abstract

A diamond spin sensor (100) includes a diamond including an NV－ center having electron spin. The transverse relaxation time of the electron spin measured by the Ramsey method is defined as Ｔ２＊ μsec. The concentration of the ＮＶ－ center in the diamond is defined as Cppm. A value α calculated by （Ｔ２＊）１／３×Ｃ using Ｔ２＊ and C is less than 2.5. In a case where the transverse relaxation time of the electron spin measured by the Hahn echo method is T2 μsec, T2 is 15 or more. The average phase difference in relation to the entire surface of the diamond is 6 nm / mm or less.

Description

Diamond Spin Sensor

[0001] The present disclosure relates to a diamond spin sensor. This application claims priority to Japanese Application No. 2024-027999, filed February 28, 2024, the entire contents of which are incorporated herein by reference.

[0002] A diamond spin sensor using the NV center of diamond (hereinafter referred to as NV center) is known. When the NV center, which is composed of nitrogen (i.e., N) in the substitution position of carbon (i.e., C) in diamond and a vacancy (i.e., V: Vacancy) adjacent to the nitrogen, becomes negatively charged, its ground state becomes a triplet state (i.e., spin S=1). The negatively charged NV center is NV － Although the term "NV center" is used in the present specification, it will be abbreviated as "NV center" below for convenience. When a charged NV center is excited by laser light with a wavelength of approximately 530 nm (i.e., green light), it emits fluorescence with a wavelength of approximately 635 nm (i.e., red light). The intensity of the fluorescence changes depending on the spin state of the NV center, and the spin state of the NV center changes due to magnetic resonance between a magnetic field applied to the NV center and microwaves or radio waves, so it can be used as a magnetic sensor.

[0003] The detection uses a diamond containing NV centers, which is a diamond spin sensor, an optical system that transmits excitation light from a light source and irradiates the NV centers, a transmission line and microwave circuit that transmit microwaves from a power source and irradiate the NV centers, and an optical system that collects fluorescence from the NV centers and transmits it to a photodetector.

[0004] Known is the method of reducing the distortion and impurity of diamond crystal, and making high-purity diamond.For example, in the following Patent Document 1, it is disclosed that, for the nitrogen element that is the largest impurity, by adding getter material such as titanium (that is, Ti) to raw material, it can make diamond with less nitrogen.In the following Patent Document 2, it is disclosed that, as the diamond of seed crystal, cut out the part with less crystal defects from the diamond of 3mm or more size to 1mm or less size, and use this as seed substrate to grow, it can reduce the distortion of crystal.

[0005] Japanese Patent Application Laid-Open No. 7-148426 Japanese Patent Application Laid-Open No. 9-165295 International Publication No. 2022 / 210723 International Publication No. 2022 / 209512 International Publication No. 2016 / 013588

[0006] The diamond spin sensor according to one aspect of the present disclosure is a NV having electron spin. － The diamond containing the center has a transverse relaxation time of electron spin measured by the Ramsey method of T2 ＊ μsec, and NV in diamond － The concentration of the center is C ppm, and T2 ＊ and C (T2 ＊ ) １／３ The value α calculated by ×C is smaller than 2.5, and where T2 μsec is the transverse relaxation time of electron spin measured by the Hahn echo method, T2 is 15 or more, and the average phase difference over the entire surface of the diamond is 6 nm / mm or less.

[0007] FIG. 1 is a perspective view showing a diamond spin sensor according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing the crystal planes and orientations of diamond. FIG. 3 is a block diagram showing the configuration of an apparatus used for measurements using the diamond spin sensor shown in FIG. 1. FIG. 4 is a sequence diagram showing the timing of irradiation of excitation light and electromagnetic waves, and the timing of measurement of synchrotron radiation, during measurements using the diamond spin sensor shown in FIG. 1. FIG. 5 is a graph showing the relationship between the observed signal intensity (i.e., fluorescence intensity) and the frequency of the electromagnetic wave (i.e., microwave). FIG. 6 is a schematic diagram showing the NV center of diamond. FIG. 7 is a sequence diagram showing the timing of irradiation of excitation light and electromagnetic waves, and the timing of measurement of synchrotron radiation, for measuring the transverse relaxation time T2 of the diamond spin sensor shown in FIG. 1. FIG. 8 is a graph showing the transverse relaxation time T2. FIG. 9 is a sequence diagram showing the transverse relaxation time T2 of the diamond spin sensor shown in FIG. 1. ＊ 1 is a sequence diagram showing the irradiation timing of excitation light and electromagnetic waves for measuring , and the measurement timing of synchrotron radiation. FIG. 10 is a graph showing the change in fluorescence intensity measured by the sequence diagram shown in FIG. 9. FIG. 11 is a schematic diagram showing a diamond synthesis apparatus. FIG. 12 is a schematic diagram showing a method for producing a seed crystal used in diamond synthesis. FIG. 13 is a schematic diagram showing a diamond synthesis method using a diamond cut out from the synthetic diamond shown in FIG. 12 as a seed crystal. FIG. 14 is a schematic diagram showing a diamond synthesis method using a seed crystal larger than the seed crystal shown in FIG. 13. FIG. 15 is a plan view of the synthetic diamond shown in FIG. 14. FIG. 16 is a plan view of a conventional synthetic diamond. FIG. 17 is a diagram showing the manufacturing conditions of the experimental sample in table format. FIG. 18 is a diagram showing the experimental results in table format.

[0008] [Problem to be Solved by the Present Disclosure] With regard to a sensor using a diamond NV center (also called a color center), the longer the transverse relaxation time T2 of the spin, the higher the sensitivity of the sensor, which is preferable. In other words, the longer the transverse relaxation time T2, the longer the time during which the resonant electromagnetic wave is applied, and therefore the higher the sensitivity of the sensor. If the transverse relaxation time T2 is long, the transverse relaxation time T2 that affects the measurement sensitivity of the measurement target (static magnetic field, etc.) of direct current (hereinafter referred to as DC (Direct Current)) ＊ The transverse relaxation time T2 and the transverse relaxation time T2 ＊ To lengthen the １３ The natural abundance of the isotope is about 98.9%. １２ C has no nuclear spin, １３ C has a nuclear spin of 1 / 2. Therefore, １３ When C exists, the electron spin of the NV center and １３ The interaction with the nuclear spin of C results in a transverse relaxation time T2 and a transverse relaxation time T2 ＊ becomes smaller. １３ To synthesize diamond with a reduced C content compared to its natural abundance, １３ By reducing the C content １２ It is necessary to use a carbon source enriched with C, which increases the number of steps required to prepare such a carbon source, which increases the manufacturing cost of the diamond used in the sensor and the product price.

[0009] The stronger the emission intensity of diamond, the easier it is to detect the signal, and the higher the sensitivity of the sensor. If the concentration of NV centers is increased to increase the emission intensity, the large amount of spin-bearing nitrogen gets in the way, shortening the transverse relaxation time T2 (leading to fast signal decay). Reducing the concentration of NV centers in diamond is also effective in lengthening the transverse relaxation time T2. However, this reduces the number of NV centers, which are the source of fluorescence, and weakens the emission intensity. In this way, there is a trade-off between the transverse relaxation time T2 and the concentration of NV centers.

[0010] As disclosed in Patent Document 1 and Patent Document 2, it is easy to make high-purity diamond crystals that are almost free of impurities and defects.However, in order to use diamond that contains NV centers that require nitrogen and defects (vacancies) as a sensor, it is necessary to remove the factors that cause scattering of fluorescence in the state where a trace amount of strain and impurities remains.That is, it is desired to find a method that reduces other impurities and strain while leaving a trace amount of nitrogen and defects (vacancies) that are impurities.

[0011] Thus, the present disclosure: １３ The object of the present invention is to provide a diamond spin sensor in which the C content remains at the natural abundance ratio and the transverse relaxation time T2 can be increased without reducing the concentration of the NV center.

[0012] [Advantages of the Present Disclosure] According to the present disclosure, １３ A diamond spin sensor can be provided in which the C content remains at the natural abundance ratio and the transverse relaxation time T2 can be increased without reducing the concentration of the NV center.

[0013] [Description of Embodiments of the Present Disclosure] The contents of the embodiments of the present disclosure will be listed and described below. At least some of the embodiments described below may be combined in any combination.

[0014] (1) The diamond spin sensor according to the first aspect of the present disclosure is a NV having electron spin. － The diamond containing the center has a transverse relaxation time of electron spin measured by the Ramsey method of T2 ＊ μsec, and NV in diamond － The concentration of the center is C ppm, and T2 ＊ and C (T2 ＊ ) １／３ The value α calculated by ×C is smaller than 2.5, and T2 is 15 or more, where T2 μsec is the transverse relaxation time of electron spin measured by the Hahn echo method, and the average phase difference over the entire surface of the diamond is 6 nm / mm or less. １３ The C content remains at its natural abundance ratio, and NV －The transverse relaxation time T2 can be increased without decreasing the concentration of the center. １２ Since there is no need to use a carbon source enriched with C, production is easy, and production costs and product prices are lower. Note that, for measuring objects (such as alternating current magnetic fields) of alternating current (hereinafter referred to as AC), the transverse relaxation time T2 ＊ It's not a problem if it's small.

[0015] (2) In (1) above, the value α may be smaller than 0.25. This reduces the transverse relaxation time T2 ＊ or NV － The concentration of centers can be smaller, but the transverse relaxation time T2 can be maintained.

[0016] (3) In the above (1) or (2), NV － The concentration of the center may be 0.002 ppm or more and less than 10 ppm, and the average phase difference may be 2 nm / mm or less. This allows the transverse relaxation time T2 to be increased, thereby realizing a sensor with higher sensitivity than conventional sensors.

[0017] (4) In any one of (1) to (3) above, T2 may be equal to or greater than 250. This makes it possible to realize a sensor with higher sensitivity than conventional sensors.

[0018] (5) In any one of (1) to (4) above, NV － The concentration of the center may be 0.002 ppm or more and less than 0.3 ppm, which can increase the transverse relaxation time T2 and realize a sensor with higher sensitivity.

[0019] (6) In any one of (1) to (5) above, T2 ＊ may be 0.05 or more and less than 0.8, which does not affect the measurement of an AC measurement target (such as an AC magnetic field).

[0020] (7) In any one of (1) to (6) above, the half-width of the rocking curve of the X-ray diffraction by the double crystal method may be less than 10 seconds, thereby increasing the transverse relaxation time T2 and realizing a highly sensitive sensor.

[0021] (8) In any one of (1) to (7) above, the half-width of the rocking curve of the X-ray diffraction by the double crystal method may be less than 6 seconds, thereby further increasing the transverse relaxation time T2 and realizing a sensor with higher sensitivity.

[0022] (9) In any one of (1) to (8) above, the number of dislocation defects detected in the entire diamond by X-ray topography imaging may be 10 or less. This allows the transverse relaxation time T2 to be increased, thereby realizing a highly sensitive sensor.

[0023] (10) In any one of (1) to (9) above, the number of dislocation defects detected in the entire diamond by X-ray topography imaging may be 0. This allows the transverse relaxation time T2 to be further increased, thereby realizing a sensor with higher sensitivity.

[0024] (11) In any one of (1) to (10) above, NV － The ratio of the concentration of isolated vacancies in the diamond to the concentration of the centers may be 10% or less, which increases the transverse relaxation time T2 and realizes a highly sensitive sensor.

[0025] (12) In the above (11), the ratio is 1% or less. This allows the transverse relaxation time T2 to be further increased, thereby realizing a sensor with higher sensitivity.

[0026] [Details of the embodiments of the present disclosure] In the following embodiments, the same components are denoted by the same reference numerals, and their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0027] Referring to FIG. 1 , the diamond spin sensor 100 according to the embodiment of the present disclosure is a rectangular parallelepiped having a first surface 102 and a first edge 104. The diamond spin sensor 100 may be a rectangular parallelepiped with equal side lengths, i.e., a cube. The diamond spin sensor 100 is formed of a single crystal diamond containing an NV center composed of nitrogen (N) and vacancies (V). The first surface 102 is a crystal plane, such as the (001) or (111) plane. When the first surface 102 is a (001) plane, the first edge 104 is formed along the <100> or <010> direction. When the first surface 102 is a (111) plane, the first edge 104 is formed along the <1-10>, <10-1>, or <01-1> direction. Note that the notation "-1" corresponds to the notation of a crystal orientation in which a bar (horizontal bar) is placed above the number 1.

[0028] Although FIG. 1 shows a rectangular parallelepiped diamond spin sensor 100, the shape is not limited to this. The shape of the first surface 102 is not limited to a rectangle and can be any shape. The first surface 102 may be, for example, a triangle. In this case, the first side 104 corresponds to one side of the triangle. The three-dimensional shape of the diamond spin sensor 100 is not limited to a rectangular parallelepiped and may be a pyramid (such as a pyramid or a cone). For example, the diamond spin sensor 100 may be a tetrahedron (e.g., a corner cube with right-angled sides) in which the first surface 102 is a triangle (e.g., an equilateral triangle).

[0029] 2, among the crystal planes of diamond, the (001) plane is a plane defined by points A5 to A8 (i.e., a plane passing through these four points). When the first surface 102 is a (001) plane as described above, the diamond spin sensor 100 is realized as a cube with vertices from points A1 to A8, for example. The <100> direction and the <010> direction, which are directions that the first edge 104 can take, are the direction from point A1 to point A2 and the direction from point A1 to point A4, respectively. For example, if the first surface 102 is a plane with vertices from points A5 to A8, the first edge 104 corresponds to the line segment connecting points A5 and A6, or the line segment connecting points A5 and A8.

[0030] The (111) plane is a plane defined by points A5, A2, and A4. As described above, the first surface 102 may be the (111) plane. In this case, the <1-10> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B1. That is, the first side 104 corresponds to the line segment connecting points A2 and A4. The <10-1> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B2. That is, the first side 104 corresponds to the line segment connecting points A5 and A2. The <01-1> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B3. That is, the first side 104 corresponds to the line segment connecting points A5 and A4.

[0031] Measurement using the diamond spin sensor 100 is performed, for example, by the device shown in Fig. 3. The control unit 230 includes a CPU (Central Processing Unit) and a storage unit (neither of which are shown). The processing performed by the control unit 230 is realized by the CPU reading and executing a program stored in advance in the storage unit.

[0032] The excitation light generating unit 210 generates excitation light for exciting the NV centers of the diamond spin sensor 100 under the control of the control unit 230. The control unit 230 supplies, for example, a voltage to the excitation light generating unit 210 at a predetermined timing to cause the excitation light generating unit 210 to emit light. The excitation light 204 is green light (i.e., wavelength 490 nm to 560 nm). The excitation light 204 is, for example, laser light, and the excitation light generating unit 210 is, for example, a semiconductor laser (e.g., emitted light wavelength 532 nm).

[0033] The filter 212 is an element for separating the excitation light 204 incident from the excitation light generating unit 210 and the light (i.e., fluorescent light) emitted from the diamond spin sensor 100. For example, the filter 212 is a filter that cuts off (i.e., reflects) light with wavelengths equal to or less than a predetermined wavelength and passes light with wavelengths greater than the predetermined wavelength, or a bandpass filter that passes light with wavelengths within a predetermined wavelength range and cuts off (i.e., reflects) light with wavelengths outside the predetermined wavelength range. Generally, excitation light has a shorter wavelength than fluorescent light, so such a configuration is preferable. For example, the filter 212 is a dichroic mirror with this function.

[0034] The focusing element 214 focuses the excitation light 204 input from the filter 212. The focusing element 214 is, for example, a spherical lens. The focusing element 214 inputs as much of the excitation light diffused and output from the excitation light generating unit 210 as possible into the end of the optical waveguide 216. The optical waveguide 216 includes a medium for transmitting light and transmits light in both directions. That is, the optical waveguide 216 has a first end and a second end, and transmits the excitation light 204 incident on the first end to the second end located near the diamond spin sensor 100. The optical waveguide 216 also transmits the emitted light (i.e., fluorescence) from the diamond spin sensor 100 incident on the second end to the first end and outputs it. The optical waveguide 216 is, for example, an optical fiber.

[0035] The LPF (Long Pass Filter) 218 ​​is a long-pass filter that passes light with wavelengths equal to or greater than a predetermined wavelength and cuts (e.g., reflects) light with wavelengths smaller than the predetermined wavelength. The fluorescent light 206, which is the emitted light of the diamond spin sensor 100, is red light and passes through the LPF 218, but the excitation light 204 output from the excitation light generator 210 has a shorter wavelength and does not pass through the LPF 218. This prevents the excitation light 204 emitted from the excitation light generator 210 from being detected by the photodetector 220 and becoming noise, which reduces the detection sensitivity of the fluorescent light 206, which is the emitted light of the diamond spin sensor 100. The photodetector 220 generates and outputs an electrical signal corresponding to the incident light. The photodetector 220 is, for example, a photodiode. The output signal from the photodetector 220 is acquired by the control unit 230.

[0036] The electromagnetic wave irradiating unit 202 irradiates the diamond spin sensor 100 with electromagnetic waves (e.g., microwaves). The electromagnetic wave irradiating unit 202 is, for example, a coil or microwave resonant circuit formed including an electric conductor. The electromagnetic waves are supplied from the electromagnetic wave generating unit 232 to the electromagnetic wave irradiating unit 202 by, for example, a coaxial cable. The irradiation of the excitation light and electromagnetic waves to the diamond spin sensor 100 is controlled by the control unit 230, and is performed, for example, at the timing shown in FIG. 4.

[0037] Referring to Fig. 4, the control unit 230 controls the excitation light generating unit 210 to output excitation light for a predetermined period (e.g., time interval t1) at a predetermined timing. The control unit 230 controls the electromagnetic wave generating unit 232 to output electromagnetic waves for a predetermined period (e.g., time interval t2) at a predetermined timing. Any appropriate pulse sequence may be used during the time interval t2. This allows the excitation light and the electromagnetic waves to be irradiated onto the diamond in a temporally and spatially combined manner. The control unit 230 captures the output signal of the light detecting unit 220 at a predetermined timing (e.g., time interval t3) and stores it in the memory unit.

[0038] The NV center transitions from the ground state to an excited state when exposed to green light with a wavelength of 490 nm to 560 nm (e.g., laser light of 532 nm), and then emits red light with a wavelength of 630 nm to 800 nm (e.g., fluorescent light of 635 nm) and returns to the ground state. The NV center captures one electron (i.e., NV － ) the magnetic quantum number m ｓ forms spin triplet states of -1, 0, and +1, and in the presence of a magnetic field, m ｓ The energy level of the state of = ±1 splits depending on the magnetic field strength (i.e., Zeeman splitting). ｓ = 0 state is m ｓAfter transitioning to the state of π = ±1 (i.e., electron spin resonance), the electron spin resonance is excited by irradiating it with green light. As a result, the transition back to the ground state includes a transition that does not emit light (i.e., fluorescence), and the intensity of the observed emitted light decreases. Therefore, a valley (i.e., a drop in the signal) is observed in the ODMR (Opticaly Detected Magnetic Resonance) spectrum.

[0039] As described above, the control unit 230 controls the excitation light generating unit 210 and the electromagnetic wave generating unit 232 to measure a spectrum such as that shown in FIG. 5. The frequency difference Δf, which is the distance between the two observed valleys, depends on the magnetic field strength at the position of the diamond spin sensor 100 (corresponding to Zeeman splitting). The control unit 230 can calculate the magnetic field from the frequency difference Δf. Referring to FIG. 6, the magnetic field detected by the NV center is a component in the direction of the axis passing through N and V of the NV center formed in the diamond (hereinafter referred to as the NV axis). That is, where φ is the angle formed by the magnetic field vector B and the NV axis, the diamond spin sensor 100 detects a change in signal intensity (i.e., fluorescence intensity) according to B cos φ.

[0040] As explained above, the magnetic field can be calculated from the change in the ODMR spectrum, but it is known that the center frequency of the two resonant frequencies of the NV center has temperature dependence in the range from 120 K to 700 K. Therefore, by using the diamond spin sensor 100 to measure the change in the center frequency split into Δf, it is possible to measure the temperature.

[0041] In the diamond spin sensor 100, the transverse relaxation time of the electron spin of the NV center is T2 ＊ μsec, the concentration of NV centers in the diamond (the ratio of the number of NV centers to the number of carbon atoms) is C ppm, and T ＊ and C are used to calculate α = (T2 ＊ ) １／３ The value α calculated by ×C is smaller than 2.5. Furthermore, assuming that the transverse relaxation time of the electron spin is T2 μsec, T2 is 15 or more, and the average phase difference over the entire surface of the diamond, which will be described later, is 6 nm / mm or less.＊ is measured by the Ramsey method, which will be described later, and the transverse relaxation time T2 is measured by the Hahn echo method, which will be described later. The concentration C of the NV center is measured by the electron spin resonance method, etc. １３ The C content remains at its natural abundance ratio, and the transverse relaxation time T2 can be increased without reducing the concentration of the NV center. Therefore, the decrease in emission intensity can be suppressed, and a sensor with higher sensitivity than conventional sensors can be realized. １２ Since there is no need to use a carbon source enriched with C, production is easy, and production costs and product prices are lower. ＊ is better for measuring DC measurement targets (static magnetic fields, etc.), but does not affect AC measurement targets (alternating current magnetic fields, etc.). Therefore, for measuring AC measurement targets (alternating current magnetic fields, etc.), it is sufficient to maintain the transverse relaxation time T2 at a certain value. ＊ It's not a problem if it's small.

[0042] (Transverse Relaxation Time T2) The Hahn echo method used to measure the transverse relaxation time T2 is included in the electron spin echo method (hereinafter referred to as the spin echo method) described below. The spin echo method is a type of ESR (Electron Spin Resonance) measurement. In a typical ESR measurement, microwaves are continuously irradiated while an external magnetic field is applied, and the absorption of the microwaves is observed (CW (Continuous Wave)-ESR). In contrast, in the spin echo method, electron spins are excited by microwave pulses and the relaxation of the electron spins is measured. The spin echo method can measure the physical quantities of the spin-lattice relaxation time T1 and the spin-spin relaxation time T2. The spin-lattice relaxation time T1 is also called the longitudinal relaxation time T1. The transverse relaxation time T2 mentioned above refers to the spin-spin relaxation time T2.

[0043] In the spin echo technique, microwaves are not applied continuously, but rather microwave pulses that rotate the spins by θ° are applied in several increments. The amount of spin rotation (i.e., rotation angle θ) is determined by the microwave intensity and application time. In many cases, the operation is θ1-τ-θ2, where the spins are rotated by θ1° using a microwave pulse, and after a time τ, the spins are rotated by θ2° using another microwave pulse. For example, θ1=90°, θ2=180°.

[0044] Consider the behavior of spins in a rotating coordinate system that rotates according to the Larmor frequency. The first pulse rotates spins aligned along the Z axis (the direction of the magnetic field) by θ1° from the Z axis toward the XY plane. All spins rotate in the same direction around the Z axis, but due to local magnetic field fluctuations, the rotation speed of each spin around the Z axis after a θ1° rotation varies slightly. Over time, the direction of each spin gradually shifts from its position immediately after the θ1° rotation. In other words, a phase delay or advance occurs in the spins. The dispersion of this shift increases in proportion to time τ (i.e., the elapsed time). If a microwave pulse is then applied that rotates the spins by θ2°, the dispersed spins rotate by θ2°, in the same direction as when the spins were previously rotated by θ1°. After the θ2° rotation, all spins still rotate in the same direction around the Z axis, and the rotation speed of each spin around the Z axis similarly differs; those that were rotating slowly now rotate slower, and those that were rotating quickly now rotate faster. However, the starting position of each spin's rotation around the Z axis (phase immediately after rotation by θ2°) is opposite to that immediately before rotation by θ2°. That is, spins with a slow rotation speed and a delayed phase become advanced, while spins with a fast rotation speed and an advanced phase become delayed. Therefore, after rotation by θ2°, the phase difference between the spins decreases over time. As a result, the spins align after time τ has elapsed. This allows a spin echo (hereinafter referred to as "echo") to be detected as a signal due to the spins.

[0045] In measuring the relaxation time of NV centers in diamond, microwave pulses are applied in the order of 90°-τ-180°-τ-90° to read the fluorescence intensity due to excited electron spins. The transverse relaxation time T2 is measured, for example, using the spin echo pulse sequence shown in Figure 7. The method of observing signals using such a pulse sequence is called the Hahn echo method. Pulse P1 and pulse P3 are each pulses that rotate the electron spin of the NV center by 90° (π / 2) as described above. Pulse P2 is a pulse that rotates the electron spin of the NV center by 180° (π) as described above. Pulses P1, P2, and P3 are applied to the diamond spin sensor 100 with the same time interval τ between them.

[0046] As mentioned above, spin echoes are generated by localized fluctuations in the magnetic field, which are caused by factors such as nonuniformity in the pulsed magnetic field, spin-nuclear interactions, and spin-spin dipole interactions. The relaxation time can be calculated from the magnitude of decay in echo intensity (fluorescence intensity) as the time interval τ is varied. Specifically, by repeatedly measuring echoes using the pulse sequence shown in Figure 7 and plotting the echo intensity (peak value of the echo signal) versus time, a graph such as that shown in Figure 8 is obtained. In Figure 8, the solid line schematically represents the measured value (fluorescence intensity). The vertical axis is expressed in arbitrary units (au). The horizontal axis is 2τ (twice the time interval τ). The dashed line represents a graph obtained by fitting the measured value graph using an exponential function. The transverse relaxation time T2 is the value of time τ when the exponential function value becomes 1 / e of its initial value. That is, the transverse relaxation time T2 represents the time for which the measurement signal is sustained, and the longer the transverse relaxation time T2, the longer the signal can be measured.

[0047] With regard to diamond spin sensors, the observed fluorescence intensity increases with the number of NV centers that generate fluorescence. However, if defects, strain, and impurities other than nitrogen that constitute the NV centers are present in the diamond single crystal, the emitted fluorescence is scattered and absorbed by them. Therefore, the greater the number of defects, strain, and impurities (other than nitrogen), the lower the measured fluorescence intensity. In other words, the measured fluorescence intensity I is considered to be inversely proportional to the amount of defects, strain, and impurities (other than nitrogen) (I ∝ 1 / X, where X represents the degree of defects, strain, or impurities (other than nitrogen)). Note that the fluorescence intensity also depends on the surface roughness of the diamond spin sensor (hereinafter referred to as surface roughness), but the influence of surface roughness can be eliminated by polishing the surface.

[0048] The transverse relaxation time T2 increases as the number of NV centers decreases. Like the fluorescence intensity, the transverse relaxation time T2 is also affected by defects, strain, and impurities (other than nitrogen) in the crystal. Defects, strain, and impurities (other than nitrogen) in the crystal act as disturbances on the transverse relaxation time T2, and it is thought that the transverse relaxation time T2 is inversely proportional to the amount of defects, strain, and impurities (other than nitrogen) (T2 ∝ 1 / X).

[0049] (Transverse relaxation time T2 ＊ ) Transverse relaxation time T2 ＊ is measured, for example, using the pulse sequence shown in Figure 9. At a predetermined timing, excitation light is irradiated onto the diamond spin sensor 100 (time interval t1), and the state of the electron spin is initialized. Then, at time interval t2, pulses P1 and P2 are applied to the diamond spin sensor 100 sequentially, with a time interval τ between them. Then, at time interval t3, excitation light is irradiated onto the diamond spin sensor 100, and the emitted light (i.e., fluorescence) from the NV center is measured. Each of pulses P1 and P2 is a pulse that rotates the electron spin of the NV center by 90° (π / 2) as described above. The method of measuring a signal using such a pulse sequence is called the Ramsey method.

[0050] When the time interval τ is changed and the pulse sequence shown in Fig. 9 is repeated to measure the fluorescence intensity, an oscillating graph such as that shown in Fig. 10 is obtained. The horizontal axis represents the time interval τ between pulses P1 and P2, and the vertical axis represents the measured value of the fluorescence intensity (arbitrary units). The oscillation frequency is determined by the difference between the microwave frequency and the resonant frequency of the NV center. The oscillation in the graph shown in Fig. 10 decays exponentially. This is because the resonant frequency of the NV center decays exponentially depending on the relationship between the NV center and the external magnetic field, temperature, and impurity spins ( １３ The transverse relaxation time T2 is determined by the vibration damping, similar to the transverse relaxation time T2. ＊ can be calculated.

[0051] Transverse relaxation time T2 ＊ The smaller the number of NV centers, the larger the transverse relaxation time T2 ＊ is affected by defects, strain and impurities (other than nitrogen) in the crystal. Defects, strain and impurities (other than nitrogen) in the crystal affect the transverse relaxation time T2 ＊ acts as a disturbance to the transverse relaxation time T2 ＊ is inversely proportional to the amount of defects, strain and impurities (other than nitrogen) (T ＊ ∝1 / X). Therefore, the transverse relaxation time T2 ＊ (μsec) and NV － The product of the center concentration C (%), specifically, α = (T2 ＊ ) １／３ The smaller the defects, strain and impurities (other than nitrogen) in the diamond crystal, the larger xC becomes.

[0052] Transverse relaxation time T2 ＊ To increase １３ Reducing the C content, that is, １２ It is effective to concentrate C. However, １３ The number of steps required to manufacture diamond, such as preparing a carbon source with a reduced C content, increases, leading to an increase in manufacturing costs. ＊ (unit: μsec) and NV －It is preferable to suppress the value α calculated from the center concentration C (unit: ppm) to less than a predetermined value. The examples described later will show that even if the value α is less than the predetermined value, a diamond spin sensor with a transverse relaxation time T2 of 15 μsec or more can be realized. In other words, α can be made smaller than 2.5. To achieve this, the average phase difference over the entire surface of the diamond spin sensor 100 described later is set to 6 nm / mm or less. This will reduce the phase difference in diamond. １３ The C content remains at its natural abundance ratio, and the transverse relaxation time T2 can be increased without reducing the concentration of the NV center, thereby realizing a sensor with higher sensitivity. α is preferably smaller than 1.5, and more preferably smaller than 0.5. α is even more preferably smaller than 0.3. α may be smaller than 0.25. α is even more preferably smaller than 0.2. Even when α is small, the transverse relaxation time T2 can be maintained.

[0053] As mentioned above, the value α depends on the number of NV centers, i.e., their concentration. － If the center concentration is between 0.002 ppm and 10 ppm, a smaller value α can be achieved than in the past. Furthermore, the average phase difference over the entire surface of the diamond spin sensor 100, which will be described later, can be reduced to 2 nm / mm or less. This allows the transverse relaxation time T2 in the diamond spin sensor to be increased. Therefore, a sensor with higher sensitivity than in the past can be realized.

[0054] The concentration of the NV center may be 0.002 ppm or more and 0.3 ppm or less, which can increase the transverse relaxation time T2 and realize a sensor with higher sensitivity.

[0055] In addition, NV in diamond －The concentration of the center can be calculated from the measurement value by, for example, electron spin resonance (CW-ESR). Also, in the case of a low concentration, it can be measured by observing with a fluorescence microscope and counting the number of single NV centers. In the case of a high concentration, for diamonds containing a low concentration of NV centers, the conversion factor between concentration and fluorescence intensity is found, and this can be used to calculate the concentration by converting from the fluorescence intensity ratio. Furthermore, the concentration of the NV center can also be calculated from the absorption coefficient at 637 nm in the absorption spectrum measured by ultraviolet-visible absorption spectroscopy.

[0056] The transverse relaxation time T2 is preferably 25 μsec or more, more preferably 80 μsec or more. The transverse relaxation time T2 may be 250 μsec or more. The transverse relaxation time T2 is more preferably 300 μsec or more. This allows for a sensor with higher sensitivity to be realized.

[0057] Transverse relaxation time T2 ＊ The transverse relaxation time T2 may be 0.05 μsec or more and less than 0.8 μsec. ＊ is preferably less than 0.6 μsec, more preferably less than 0.5 μsec, and even more preferably less than 0.25 μsec, so that there is no effect on the measurement of an AC measurement target (such as an alternating magnetic field).

[0058] (Average Phase Difference) The average phase difference will now be explained. Diamonds inherently have an isotropic crystal structure and an isotropic refractive index (dielectric constant). However, in reality, defects and distortions exist in single-crystal diamonds, causing diamonds to have birefringence. When circularly polarized light is irradiated onto a diamond with birefringence, a phase difference occurs between two orthogonal polarized light (linearly polarized light), resulting in the output of elliptically polarized light. The optical axis and phase difference can be determined from the orientation of the major and minor axes of the ellipse in the elliptically polarized light output from the diamond, as well as the ratio of the lengths of the major and minor axes. The measured phase difference is a value integrated in the direction in which light passes through the diamond (e.g., the thickness direction of the diamond). Therefore, the phase difference is normalized by, for example, the thickness of the diamond, i.e., expressed as a phase difference converted to a thickness of 1 mm (unit: nm / mm). The phase difference is measured locally and distributed two-dimensionally within the measurement surface. Therefore, the phase difference is expressed as an average value within the measurement surface (hereinafter referred to as the average phase difference). The term "average value" does not mean the phase difference per area, but rather means the average value of the phase differences obtained by performing local measurements multiple times over the plane, i.e., the average of the frequency distribution of the phase difference over the plane.

[0059] As described above, the average phase difference over the entire surface of the diamond spin sensor 100 can be 6 nm / mm or less. Also, as described above, the average phase difference over the entire surface of the diamond spin sensor 100 may be 2 nm / mm or less. This allows the transverse relaxation time T2 to be further increased, resulting in a sensor with higher sensitivity.

[0060] (Half-width of rocking curve) In X-ray diffraction using the double crystal method described below, another diamond crystal cut from the original crystal from which the diamond spin sensor 100 shown in Figure 1 was cut is used as the first crystal, and the diamond spin sensor 100 is used as the second crystal, and measurements are taken using CuKα radiation in a (004) plane parallel arrangement. The half-width (angle) of the rocking curve of the diamond spin sensor 100 obtained from the measurement may be less than 10 arcsec. This allows the transverse relaxation time T2 to be increased. Therefore, a sensor with higher sensitivity than conventional sensors can be realized. Note that half-width means full width at half maximum.

[0061] The crystallinity of diamond can be evaluated by measuring the rocking curve in X-ray diffraction. The smaller the half-width of the X-ray diffraction rocking curve, the higher the crystallinity of the diamond and the fewer crystal defects and distortions. To improve detection accuracy, the double-crystal method is used. In the double-crystal method, two crystals are used: an analyzing crystal (first crystal) and a sample crystal (second crystal) to be evaluated. X-rays output from an X-ray source are irradiated onto the first crystal, the resulting diffracted X-rays are incident on the second crystal, and the angle of the second crystal is changed to measure the diffracted X-rays with a detector. The angle of incidence of the diffracted X-rays on the second crystal (the angle between the second crystal and its plane) is represented by ω, and the angle between the direction of the detector and the direction of incidence of the diffracted X-rays on the second crystal is represented by 2θ. By fixing the first crystal and the detector and placing the center of rotation on the surface of the second crystal and rotating the second crystal, the diffracted X-rays are measured by fixing the angle 2θ and varying only the angle of incidence ω. This allows the measurement of the diffraction intensity distribution on a spherical surface centered on the origin in reciprocal lattice space. A rocking curve is obtained by plotting the detected values ​​of the measurement results on the vertical axis with the incident angle ω on the horizontal axis, and the width (angle) of the rocking curve is proportional to the degree of fluctuation in the plane orientation. By using crystals cut from the same single diamond crystal for the first and second crystals, the half-width (angle) of the rocking curve sensitively reflects the quality of the crystal being measured.

[0062] In the diamond spin sensor 100, the half-width of the X-ray diffraction rocking curve obtained by the double crystal method is preferably less than 8 seconds, and more preferably less than 7 seconds. The half-width of the X-ray diffraction rocking curve obtained by the double crystal method may be less than 6 seconds. The half-width of the X-ray diffraction rocking curve obtained by the double crystal method is even more preferably less than 5 seconds. This allows the transverse relaxation time T2 to be further increased. Therefore, a sensor with higher sensitivity than conventional sensors can be realized.

[0063] In the diamond spin sensor 100 shown in FIG. － The ratio of the concentration of isolated vacancies to the concentration of centers may be 10% or less. Isolated vacancies refer to vacancies in a state where no nitrogen exists around the vacancies. Isolated vacancies include uncharged vacancies and negatively charged vacancies. Therefore, the concentration ratio is NV － It can be calculated as the ratio of the number of uncharged vacancies and the number of negatively charged vacancies to the number of centers. This allows the transverse relaxation time T2 to be increased, resulting in a highly sensitive sensor. The density (concentration) of vacancies can be calculated by the ratio of the number of neutral isolated vacancies (V ０ ) and the density of negatively charged isolated vacancies (V － The density of each isolated vacancy is calculated by the integrated absorption of light at wavelengths of 741 nm and 394 nm (wavelength integral of absorption coefficient: unit: meV × cm) at liquid nitrogen temperature. －１ ) can be calculated from the integral absorption A and density β (unit cm －３ ) and the proportionality coefficient k (k = A / β) is 1.2 × 10 －１６ , and 4.8 × 10 －１６ In addition, when the vacancy density cannot be calculated using the absorption coefficient in the visible region, it can be calculated using the positron annihilation method. The relative density value obtained by the positron annihilation method can be converted into density even in low concentration regions by calibrating (proportional calculation) using the value in the region where density can be obtained using the absorption coefficient as a reference.

[0064] In the diamond spin sensor 100, NV －The ratio of the concentration of isolated vacancies to the concentration of centers is preferably 5% or less, and more preferably 3% or less. － The ratio of the concentration of isolated vacancies to the concentration of centers may be 1% or less, which can increase the transverse relaxation time T2 and realize a sensor with higher sensitivity.

[0065] (Dislocation defects) As mentioned above, when the concentration of NV centers is 0.01 ppm or more and less than 1.0 ppm, the number of dislocation defects detected in the entire diamond spin sensor 100 by X-ray topography images needs to be 10 or less. Dislocation defects refer to defects in which the alignment of one or more crystals in the diamond is misaligned, forming a linear boundary with the non-misaligned portion. If there are fewer dislocation defects in the diamond, the transverse relaxation time T2 can be increased, and a highly sensitive sensor can be realized.

[0066] Growth sectors and dislocation defects in diamond can be detected by X-ray diffraction. That is, MoKα1 X-rays (characteristic X-rays of molybdenum with a wavelength λ = 0.71 Å (0.071 nm)) are used as X-rays, and X-ray topography images are taken using a Lang camera due to diffraction from the (220) plane of single-crystal diamond. To obtain dislocation defects throughout the diamond, X-ray topography images are taken across the entire diamond substrate, and linear defects are counted. To make the distribution of defects more easily visible, it is preferable to process the sample into a thin plate with a thickness of about 0.5 mm. For example, a single-crystal diamond is cut into a thin plate using a laser processing machine, and the cut surface is flattened by skiff polishing. Alternatively, a slit can be placed next to the X-ray source to obtain X-ray topography images using diffracted rays from only a limited layer within the sample (tomographic topography: limited projection topography). This allows a relatively thick sample to be evaluated for defects without processing it into a thin plate.

[0067] The number of dislocation defects detected in the entire diamond by X-ray topography images is preferably 8 or less, more preferably 7 or less, and even more preferably 3 or less. The number of dislocation defects detected in the entire diamond by X-ray topography images may be 0 (i.e., no dislocation defects are detected). This allows the transverse relaxation time T2 to be further increased, resulting in a more sensitive sensor.

[0068] (Manufacturing Method of Diamond Spin Sensor) A method for manufacturing the diamond spin sensor 100 shown in Fig. 1 will now be described. Granular diamond is used as a seed crystal to produce a synthetic diamond by a temperature difference method under high pressure, and a portion of this is selected and cut out to be used as a seed crystal in the subsequent process.

[0069] The configuration of an apparatus for synthesizing diamond by the temperature difference method under high pressure is shown in Figure 11. In the temperature difference method, crystals are grown by utilizing the difference in solubility of diamond in a solvent caused by a temperature difference. Referring to Figure 11, a vertical temperature gradient is formed in a pressure medium 250 equipped with a graphite heater 252 and an insulating member 254, with the insulating member 254 placed in the high-temperature section and the seed crystal 300 placed in the low-temperature section, and a solvent metal 258 placed between them. Single-crystal diamond is grown on the seed crystal 300 by maintaining conditions above the pressure at which diamond becomes thermally stable at a temperature above the temperature at which the solvent metal 258 melts. Diamond powder is preferably used as the carbon source 256. Graphite or pyrolytic carbon may also be used as the carbon source 256. The carbon source 256 may be, １２ No C enrichment was performed, and the carbon source 256 １２ C and １３The ratio of C is the same as that found in nature. Solvent metal 258 is made of one or more metals selected from iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), etc., or an alloy containing these metals. Pressure is applied by pressure medium 250, which receives external force from a high-pressure generator (not shown), and heating is performed by graphite heater 252, achieving a pressure at which diamond is thermodynamically stable and a temperature condition at which solvent metal 258 and carbon are eutectic. Carbon dissolves into solvent metal 258 from carbon source 256 in the high-temperature section and is transported by diffusion to the low-temperature section below solvent metal 258, whereupon a crystal grows on seed crystal 300, forming synthetic diamond 302.

[0070] (Step 1) First, a single crystal diamond with a nitrogen concentration of 30 ppm or less is prepared. In the above-mentioned high-pressure single crystal diamond synthesis method, a single crystal diamond with a nitrogen concentration of 30 ppm or less can be produced by adding a nitrogen getter to the solvent metal 258. This allows a seed crystal with a single growth sector and reduced defects, with 10 or fewer dislocation defects detected by X-ray topography, to be cut out in the second step described below. The growth sector can be confirmed by a two-dimensional image (surface distribution image) of PL (photoluminescence: light emission image due to ultraviolet irradiation) or CL (cathodoluminescence: light emission image due to electron beam irradiation) fluorescence. In other words, it can be determined by whether the boundary between regions with different fluorescence intensity is linear (see Patent Document 3).

[0071] Dislocations are measured, for example, by an etching test (see Patent Document 4). The etching test is carried out as follows: A single crystal diamond is immersed in an etching solution of potassium nitrate (KNO ３ The diamond is immersed in the melt and heated to 600°C in a platinum crucible for 1 hour (etching). After slow cooling, the single crystal diamond is removed and the surface is observed under an optical microscope at 50x magnification. The number of point-like etch pits is counted within a rectangular measurement area of ​​1000µm x 1000µm, and the number of pits is counted within 1mm. ２The number of point-like etch pits per 1 mm can be obtained. Point-like etch pits refer to point-like depressions present on the surface of a single crystal diamond. Point-like etch pits correspond to dislocation defects. Point-like depressions are square, square with rounded corners, or approximately circular on the (100) face of a single crystal diamond, and are triangular, triangular with rounded corners, or approximately circular on the (111) face. The diameter of the point-like depressions is approximately 1 μm to 50 μm. ２ By multiplying the number of punctate etch pits per cm by 100, ２ The number of point-like etch pits (dislocation density) per 1000 nm is calculated. Note that linear etch pits may be observed on the surface of the single crystal diamond after etching, along with point-like etch pits. Linear etch pits are derived from stacking faults in the single crystal diamond. The number of linear etch pits is not included in the measurement of dislocation defects.

[0072] Dislocations can also be detected by X-ray topography (see Patent Document 5). When measuring in transmission using synchrotron X-rays, for example, X-rays with a wavelength of 0.71 Å (0.071 nm) are used, and (220) diffraction at a diffraction angle 2θ = 32.9° is used. When measuring in reflection, for example, X-rays with a wavelength of 0.96 Å (0.096 nm) and (113) diffraction at a diffraction angle 2θ = 52.4° may be used. Images may be taken by changing the wavelength of the X-rays and the diffraction angle 2θ. Laboratory X-ray diffraction equipment may also be used for measurement; for example, a Mo source may be used to observe (111) diffraction, and a Cu source may be used to observe (113) diffraction. While a CCD (Charge Coupled Device) camera can be used for measurement, it is preferable to use a nuclear emulsion plate to increase resolution. After developing the nuclear emulsion, dislocations can be identified and quantified by capturing images with an optical microscope.

[0073] In the synthesis of single-crystal diamond using the temperature difference method, for example, the composition of the solvent metal 258 is Fe / Co = 10 / 90 to 90 / 10 (mass ratio), and titanium (Ti) or aluminum (Al) is added to the solvent metal as a nitrogen getter at 1.5 mass% to 3 mass%. The temperature gradient is adjusted so that the temperature difference between the carbon source 256 and the seed crystal 300 is 10°C to 25°C, and a pressure of 5.0 GPa to 5.5 GPa and a temperature of 1300°C to 1350°C are maintained for 80 hours to 250 hours. As a result, referring to Figure 12, synthetic diamond 302 is synthesized from the seed crystal 300. If the temperature difference exceeds 25°C, crystal growth becomes disrupted and vicinal growth marks tend to become obscured. If the temperature difference is lower than 10°C, it takes a long time to grow a crystal of a predetermined size, which increases production costs. Furthermore, the temperature change during the holding period is controlled to within 3°C. This further improves the crystallinity. If the temperature change is more than 3°C, the growth becomes unstable, and crystal defects, distortion, and inclusions occur, resulting in a decrease in crystallinity.

[0074] (Step 2) From the single crystal diamond synthesized in Step 1, a cut diamond 304 (see FIG. 12) is cut out to be used as a seed crystal in Step 3 described later. The seed surface of the cut diamond 304 used as the seed crystal is preferably, but not limited to, a square or octagon. The size of the seed surface (e.g., the length of the opposite side) is preferably 0.3 mm or more and 3 mm or less. The seed crystal is cut out by laser processing into a plate with a thickness of about 0.5 mm to 1.0 mm, and the surface of the plate is polished to a surface roughness Ra of 20 nm or less. Then, it is preferably cut out by laser cutting into a plate with a size of about 0.3 mm x 0.3 mm x 0.3 mm to 3.0 mm x 3.0 mm x 1.0 mm. A larger size is preferable because it avoids dislocation defects and makes it easier to obtain a single sector. If the cut diamond 304 is a rectangular parallelepiped and the plane serving as the seed surface is a (001) crystal plane, one side of the rectangular seed surface is parallel to the <100> or <010> direction. If the plane serving as the seed surface is a (111) crystal plane, one side of the rectangular seed surface is parallel to the <1-10>, <10-1>, or <01-1> direction. This allows the cutting margin to be small, resulting in a seed crystal with a seed surface that is less damaged.

[0075] In single crystal diamond synthesized by the temperature difference method, there are many dislocation defects in the direction of the main surface of the seed crystal and in the direction of the opening angle of about X° from that direction.For example, when the main surface direction is the <001> direction, there are many dislocation defects in the four directions of the <112> direction, the <-112> direction, the <1-12> direction and the <-1-12> direction, which have an opening angle of about 35° (i.e., X=35).In addition, when the main surface direction is the <111> direction, there are many dislocation defects in the three directions of the <112> direction, the <121> direction and the <211> direction, which have an opening angle of about 19.5° (i.e., X=19.5).In the above description, the opening angle is the X° direction, because when the main surface direction and the <001> direction or the <111> direction do not coincide, the opening angle will be slightly different, so it is expressed as X°.When they do not coincide, the opening angle will be an angle corrected from 35° or 19.5°. Excluding these opening angles and the ±5° direction of the main surface, a single sector contains a high-quality crystal with few crystal defects. Furthermore, near the boundaries of different sectors, such as the boundary between the {001} sector and the {111} sector, there is a lot of crystal distortion and defects are likely to occur. As the seed substrate becomes larger, the spacing between the opening angles starting from the seed substrate becomes wider, and the boundaries of different sectors are also biased toward the edges, so the size of the high-quality crystal can also be increased. Here, the {abc} sector refers to a region grown by stacking on the {abc} plane. For example, the {001} sector is a region grown by stacking {001} planes. Therefore, in this second step, a seed crystal with few defects can be obtained by cutting out a cut diamond 304 from a single growth sector of the main surface growth of the synthetic diamond 302, excluding ±5° of the main surface growth direction of the seed crystal.

[0076] As the seed substrate becomes larger, the size of the high-quality crystal can also be increased. For example, a seed crystal with reduced defects, with 10 or less dislocation defects detected by X-ray topography, can be cut out. By using this seed crystal, the single crystal diamond synthesized in the subsequent process will have fewer defects and further reduced distortion. The single growth sector of the primary surface growth of the synthetic diamond 302, excluding ±5° from the primary surface growth direction of the seed crystal, can be, for example, the {001} sector or the {111} sector. In addition, any other crystallographically possible sector (such as the {113} sector, the {115} sector, the {110} sector, or the {135} sector) can also be used, as long as it is a single growth sector of the primary surface growth, excluding ±5° from the primary surface growth direction of the seed crystal. When cutting out the cut diamond 304 used as the seed crystal from the synthetic diamond 302, it can be cut out from within one sector, or it can be cut out to include two or more sectors, as long as it does not include a portion with many dislocation defects. That is, the cut diamond 304 may include zero or one sector boundary. In a single crystal diamond, the region of each sector can be identified by an luminescence image obtained by irradiating it with ultraviolet light (ultraviolet-excited luminescence image).

[0077] (Step 3) Diamond is synthesized by the temperature difference method as described above using the seed crystal cut out in Step 2. That is, referring to Fig. 13, cut diamond 304 is used as seed crystal 310 to synthesize synthetic diamond 312. This makes it possible to obtain a single crystal diamond with reduced crystal defects and strain.

[0078] Specifically, referring to FIG. 11 , diamond powder is used as the carbon source 256. Iron or cobalt, which has high solubility and affinity for carbon, is used as the solvent metal 258. Depending on the synthesis conditions, trace amounts of nickel or manganese may be incorporated into the diamond. The amount of boron impurity contained in the carbon source 256 and the solvent metal 258 is controlled to 1 ppm or less. This allows the boron (B) atomic number content of the single crystal diamond to be 0.1 ppm or less. Titanium is added to the solvent metal 258 as a nitrogen getter. The concentration of the added titanium is 1.5 mass% or more and 3 mass% or less. This allows the nitrogen content of the single crystal diamond to be 0.1 ppm or more and 10 ppm or less, based on the atomic number. Aluminum may also be added as a nitrogen getter. In this case, an Fe-Al alloy may also be used for the solvent metal 258.

[0079] Conditions for the temperature difference method include, for example, adjusting the temperature gradient so that the temperature difference between the carbon source 256 and the seed crystal 300 is 10°C or more and 25°C or less, and maintaining a pressure of 5.0 GPa or more and 5.5 GPa or less and a temperature of 1300°C or more and 1350°C or less for 80 hours or less and 300 hours or less. If the temperature difference exceeds 25°C, crystal growth becomes somewhat disturbed, and vicinal surface growth often becomes impossible to observe. Furthermore, by controlling the temperature change during maintenance to within 3°C, crystallinity is further improved.

[0080] (Step 4) Cut diamonds 318 are cut from the synthetic diamond 312 (see FIG. 13) synthesized in Step 3. The cut diamonds 318 are located within a single growth sector (e.g., sector 316) that does not include the sector boundary 314, and are regions with 10 or fewer dislocations detected by X-ray topography. The cut diamonds 318 are cut using laser processing in the same manner as in Step 2 described above.

[0081] (Step 5) The cut diamond 318 cut from the synthetic diamond 312 in step 4 is irradiated with an electron beam having an energy of 300 KeV or more and 1.2 MeV or less. This ionizes the orbital electrons of carbon in the cut diamond 318, ejecting carbon nuclei and forming vacancies in the cut diamond 318. Alternatively, an electron beam having an energy of 500 KeV or more and 1 MeV or less may be irradiated. The electron beam irradiation dose is determined based on the amount of NV desired to be generated. － 1 x 10 depending on the quantity １８ cm －２ From 4 x 10 １９ cm －２ The temperature can be varied in the range of

[0082] (Step 6) The cut diamond 318 that has been subjected to Step 5 is annealed in a vacuum at a temperature of 1100° C. to 1400° C. for 0.1 to 0.5 hours, thereby moving the nitrogen in the cut diamond 318 and forming NV centers consisting of nitrogen and vacancies.

[0083] For the above reasons, １２ C and １３ By using a carbon source 256 in which the ratio of C is the same as that in nature, it is possible to manufacture a diamond spin sensor 100 that is larger than conventional ones and has an increased transverse relaxation time T2 without reducing the NV center concentration C. Therefore, it is possible to suppress the decrease in fluorescence intensity and realize a sensor with higher sensitivity.

[0084] The irradiation step in the fifth step and the annealing step in the sixth step may be repeated two or more times to form a desired amount of NV centers in the cut diamond 318.

[0085] As described above, a nitrogen getter is used in the third step. The nitrogen getter can include any of titanium (Ti), zirconium (Zr), hafnium (Hf), aluminum (Al), gallium (Ga), copper (Cu), silver (Ag), and gold (Au). This can increase the transverse relaxation time T2 and realize a more sensitive sensor.

[0086] As mentioned above, the cut diamond 318 may include only one or no sector boundaries, which can increase the transverse relaxation time T2 and result in a more sensitive sensor.

[0087] 14, the above synthesis step (see step 3) may be performed using a larger seed crystal 320. For example, in a rectangular parallelepiped seed crystal 320, the length L of one side of the rectangle of the (001) crystal face or the (111) crystal face is 3 mm or more. This allows the sector boundary 324, which is prone to impurities, to be separated from the center of the sector 326. Therefore, a larger cut diamond 328 can be cut from the sector 326 of the synthetic diamond 322 grown from the seed crystal 320. The cut cut diamond 328 is then subjected to electron beam irradiation (step 5) and annealing (step 6) as described above. This allows a larger diamond spin sensor 100 to be manufactured.

[0088] The diamond synthesized as described above is shown schematically in FIG. 15 . FIG. 15 is a plan view of the (001) plane. The left-right direction in FIG. 15 corresponds to the <100> direction. The triangular sectors 332 at the four corners represent (111) sectors. The boundaries between different sectors are sector boundaries with different impurity concentrations (see dot pattern), forming distortions and introducing impurities. As shown in FIG. 14 , by enlarging the seed crystal, the impurity-infused region can be positioned more toward the periphery from the center. Therefore, a larger region can be cut out to produce a larger diamond spin sensor. Referring to FIG. 15 , a diamond spin sensor can be fabricated by cutting out the shaded portion of one sector 330 and using it as a seed crystal. Alternatively, as shown by the dashed lines, a larger seed crystal can be fabricated by cutting out a diamond including dislocation defects and two sector regions, and then using that to fabricate a diamond spin sensor. Furthermore, the entire (111) sector 330 can be used as a seed crystal to fabricate a diamond spin sensor.

[0089] As a comparative example, a synthetic diamond for a conventional NV spin sensor is shown in Figure 16. Figure 16 is a plan view similar to Figure 15. Sector 342 is a (111) sector. In the synthetic diamond of Figure 16, a large sector boundary (see dot pattern) with different impurity concentrations is formed around the periphery of the largest (001) sector 340, and it can be seen that it is not possible to cut out an area large enough for a diamond spin sensor. This is because different sectors, such as the (001) sector and the (111) sector, form a boundary.

[0090] The effectiveness of the diamond spin sensor of the present disclosure will be demonstrated below by way of examples. Using a plurality of diamond spin sensors fabricated by the above-described manufacturing method, the transverse relaxation times T2 and T2 ＊ and the NV center concentration C. The manufacturing conditions are shown in FIG. 17, and the measurement results are shown in FIGS.

[0091] In Figure 17, diamond seed crystals of a predetermined size were cut out from the synthetic diamond produced as described above using a laser processing machine (see step 2) and are shown as Samples 1 to 8. For example, Samples 1, 2, 5, and 6 are seed crystals of 3.0 mm x 3.0 mm x 1.0 mm. Sample 9 is a commercially available synthetic diamond (DNV-B14 manufactured by ElementSix) produced by CVD (Chemical Vapor Deposition). Samples 7 and 8 were cut out from diamonds produced under conditions with a relatively high nitrogen concentration, and are positioned as comparative examples along with Sample 9. The various crystals were cut out from the sectors listed in the "Seed Crystal Cutting Sector" column of Figure 17. In the "Seed crystal cutting sector" column, "(100)" means cutting from the (100) sector. Also, "(100) + (111)" means cutting to include the (100) sector and the (111) sector. The presence or absence of defects in the obtained diamond seed crystal was confirmed using an X-ray topography image. The number of confirmed dislocation defects is shown in the "Dislocation Defects" column of Figure 17. The number of dislocation defects in Samples 1 to 6 is much smaller than in Samples 7 and 8. This is due to the low nitrogen concentration of the diamond seed crystal.

[0092] Next, a diamond crystal was grown on the diamond seed crystal by the temperature difference method to obtain a single crystal diamond for each sample (see step 3). This is indicated by HPHT (High-Pressure High-Temperature) in Figure 17. Diamond powder containing 100 ppm to 200 ppm of nitrogen and 0.5 ppm to 1 ppm of boron as impurities was used as the carbon source. High-purity iron (Fe) and cobalt (Co) were used as the solvent metal, with a solvent composition of Fe:Co = 55:45 (weight ratio). 1.75 mass% titanium was added to the solvent metal. The temperature difference method for each sample was performed under the following conditions: titanium was used as a nitrogen getter, the temperature difference between the high-temperature section (carbon source) and the low-temperature section (seed crystal) was 23°C, the pressure was 5.3 GPa, and the holding temperature of the low-temperature section was 1350°C. The holding time was 150 hours.

[0093] Thereafter, the above-described fourth to sixth steps were carried out to fabricate a plurality of diamond spin sensors. The electron beam irradiation energy was 0.95 MeV, and the electron beam irradiation dose was 1×10 １８ cm －２ The annealing temperature was 950°C and the annealing time was 60 minutes. The diamonds cut by the fourth step are shown in the "Method of cutting out the sensor material" column in Figure 17. Regarding the "Position of the growth sector", "above" represents the shaded area in Figure 15, and "above + adjacent" represents the dashed area in Figure 15. The measured nitrogen concentration (unit: ppm) and NV － The center concentration (ppm) is shown in the "N concentration" and "NV" columns, respectively. － The N concentration and NV concentration of Sample 1 to Sample 6 are shown in the "Concentration" column. － The concentrations are all smaller than those of Sample 7 and Sample 8.

[0094] The transverse relaxation time T2 and the transverse relaxation time T2 ＊ The results are shown in Figure 18. Samples 1 to 9 are the same as those in Figure 17. In Figure 18, the phase difference indicates the average phase difference (unit: nm / mm) over the entire surface. Xrc indicates the half-width (unit: arcsec) of the rocking curve of X-ray diffraction. (V ０ +V－ ) / NV － is NV － α is the ratio of the sum of the number of uncharged isolated vacancies and the number of negatively charged isolated vacancies to the number of centers. ＊ (μsec unit) and NV － Using the center concentration C (ppm unit), α = (T2 ＊ ) １／３ × C.

[0095] As shown in FIG. 18, Samples 1 to 6 all have a smaller product α and a larger transverse relaxation time T2 than Samples 7 to 9. － The center concentration is affected. That is, NV of Sample 1 to Sample 6 － The center concentration is NV from Sample 7 to Sample 9. － Therefore, the transverse relaxation times T2 of Samples 1 to 6 are longer than the transverse relaxation times T2 of Samples 7 to 9.

[0096] With regard to the product α, α<2.5 was achieved for all of Sample 1 to Sample 6. With regard to Sample 6, α<0.25 was achieved. With regard to the transverse relaxation time T2, T2≧15 was achieved for all of Sample 1 to Sample 6. With regard to Sample 6, T2≧250 was achieved. It can be seen that with regard to Sample 1 to Sample 6, the smaller the product α, the longer the transverse relaxation time T2 that was achieved.

[0097] Transverse relaxation time T2 ＊ Regarding NV, Sample 4 and Sample 5 achieved NV of 0.05 μsec or more and less than 0.8 μsec. － Regarding the center concentration, Sample 1 to Sample 6 all achieved a concentration of 0.002 ppm or more and 10 ppm or less. Sample 6 achieved a concentration of 0.02 ppm or more and 0.3 ppm or less. (V ０ +V － ) / NV －Regarding the above, all of Sample 1 to Sample 6 were able to achieve 10% or less, and Sample 1 and Sample 6 were able to achieve 1% or less.

[0098] With regard to the average phase difference over the entire surface, all of Samples 1 to 6 were able to achieve 6 nm / mm or less. With regard to Samples 1 and 6, an average phase difference of 2 nm / mm or less was achieved. With regard to the half-width of the rocking curve of X-ray diffraction by the double crystal method, all of Samples 1 to 6 were able to achieve less than 10 seconds. With regard to Samples 1, 5, and 6, less than 6 seconds was achieved. With regard to the number of dislocation defects (see FIG. 17), all of Samples 1 to 6 were able to achieve 10 or less, and with regard to Samples 5 and 6, 0 was achieved.

[0099] Although the present disclosure has been described above by explaining the embodiments, the above-described embodiments are merely examples, and the present disclosure is not limited to only the above-described embodiments. The scope of the present disclosure is defined by the claims in the scope of the claims, taking into consideration the description of the detailed description of the invention, and includes all modifications within the meaning and scope equivalent to the wordings described therein.

[0100] 100 Diamond spin sensor 102 First surface 104 First edge 202 Electromagnetic wave irradiating section 204 Excitation light 206 Fluorescence 210 Excitation light generating section 212 Filter 214 Light-collecting element 216 Optical waveguide 218 LPF 220 Light detecting section 230 Control section 232 Electromagnetic wave generating section 250 Pressure medium 252 Graphite heater 254 Insulating member 256 Carbon source 258 Solvent metal 300, 310, 320 Seed crystal 302, 312, 322 Synthetic diamond 304, 318, 328 Cut diamond 314, 324 Sector boundary 316, 326, 330, 332, 340, 342 Sector A1, A2, A3, A4, A5, A6, A7, A8 Point C Carbon L Length N Nitrogen P1, P2, P3 Pulse t, t1, t2, t3, τ Time interval V Hole X, Y, Z axis Δf Frequency difference φ Angle

Claims

1. NV with electron spin － The diamond includes a center, and the transverse relaxation time of the electron spin measured by the Ramsey method is T2 ＊ μsec, and the NV in the diamond － The center concentration is C ppm, and the T2 ＊ and using the C (T2 ＊ ) １／３ x C is smaller than 2.5, T2 is the transverse relaxation time of the electron spin measured by the Hahn echo method, where T2 is 15 or more, and the average phase difference over the entire surface of the diamond is 6 nm / mm or less.

2. The diamond spin sensor according to claim 1, wherein the value α is less than 0.

25.

3. The above NV － 3. The diamond spin sensor according to claim 1, wherein the concentration of the centers is equal to or greater than 0.002 ppm and less than 10 ppm, and the average phase difference is equal to or less than 2 nm / mm.

4. A diamond spin sensor according to any one of claims 1 to 3, wherein T2 is 250 or more.

5. The above NV － 5. The diamond spin sensor according to claim 1, wherein the concentration of the center is equal to or greater than 0.002 ppm and less than 0.3 ppm.

6. The above T2 ＊ 6. The diamond spin sensor according to claim 1, wherein is equal to or greater than 0.05 and less than 0.

8.

7. A diamond spin sensor according to any one of claims 1 to 6, wherein the half-width of the rocking curve of X-ray diffraction by the double crystal method is less than 10 seconds.

8. A diamond spin sensor according to any one of claims 1 to 7, wherein the half-width of the rocking curve of X-ray diffraction by the double crystal method is less than 6 seconds.

9. A diamond spin sensor according to any one of claims 1 to 8, wherein the number of dislocation defects in the entire diamond detected by X-ray topography images is 10 or less.

10. A diamond spin sensor according to any one of claims 1 to 9, wherein the number of dislocation defects detected in the entire diamond by X-ray topography imaging is zero.

11. The above NV － 11. The diamond spin sensor according to claim 1, wherein the ratio of the concentration of isolated vacancies in the diamond to the concentration of the center is 10% or less.

12. The diamond spin sensor according to claim 11, wherein said percentage is 1% or less.

Images