Magnetic anisotropy quantitative method and system for detecting terahertz signal based on rotating magnetic field

Abstract

The invention discloses a magnetic anisotropy quantitative method and system for detecting terahertz signals based on a rotating magnetic field, and the method comprises the steps: (1), obtaining a to-be-detected magnetic / non-magnetic material film which comprises at least one non-magnetic film layer with strong spin-charge flow conversion and at least one magnetic film layer; (2) transmitting femtosecond laser pumping pulse to the magnetic / non-magnetic material film to be detected, and radiating terahertz waves; (3) applying a magnetic field to the magnetic / non-magnetic material film to be tested; (4) setting the relative time delay of the femtosecond laser detection pulse and the femtosecond laser pumping pulse to the signal position of the terahertz wave, continuously rotating the angle of applying the magnetic field, and detecting terahertz wave signals at different magnetic field angles through the femtosecond laser detection pulse; and (5) calculating to obtain the magnetic anisotropy field of the magnetic thin film layer according to the terahertz wave signals under different magnetic field angles. The method does not need to contact a sample for measurement, does not need to carry out micro-nano processing on a detection material, is simple in preparation and test processes, and can realize nondestructive detection.

Description

Technical Field

[0001] The present invention relates to a magnetic anisotropy quantification technique, and in particular, to a magnetic anisotropy quantification method and system based on detecting terahertz signals using a rotating magnetic field. Background Art

[0002] Magnetic anisotropy refers to the anisotropy existing in a magnetic material during the magnetization process along different magnetization orientations. As a basic magnetic property in magnetic materials, it has currently been widely used in magnetic memories, magnetic sensors, and related devices. In magnetic storage devices represented by giant magnetoresistance, tunneling magnetoresistance, and spin-transfer torque, an antiferromagnetic layer is usually used to induce unidirectional magnetic anisotropy to achieve the "switching" of resistance; magnetic topological structures represented by skyrmions are expected to achieve efficient information transmission and storage, and the generation of their topological magnetic structures is closely related to perpendicular magnetic anisotropy; two-dimensional van der Waals materials stand out in high-density information storage due to their atomic layer thickness. According to the Mermin–Wagner theorem, a two-dimensional finite-length Heisenberg model does not generate spontaneous magnetization, and inducing strong magnetic anisotropy can achieve room-temperature magnetism of two-dimensional materials. Therefore, developing a quantification method for magnetic anisotropy in magnetic systems is crucial for the research and development of high-density, fast-response, and low-power consumption information storage and sensor devices.

[0003] Currently, there are many methods for quantifying magnetic anisotropy in the field of spintronics, such as quantifying magnetic anisotropy by the magnetic torque method through magnetic, electrical transport properties, and magneto-optical Kerr effect; quantifying magnetic anisotropy by the change in ferromagnetic resonance and Brillouin scattering frequencies, etc. The magnetic torque method is one of the important methods for measuring magnetic anisotropy. Under the condition of ensuring the consistent rotation of a single domain in the system, the magnitude of magnetic anisotropy is quantified by measuring the magnetic properties of a sample using a rotating magnetic field; the latter quantifies the magnitude of the anisotropy field by measuring the frequency response of a magnetic sample after being excited by a microwave or optical field and combining the Landau–Lifshitz–Gilbert equation. The above experimental measurement methods are mainly static or quasi-static experimental measurement methods, and some require contact measurement during the measurement process, or the sample to be measured needs to be micro-nano processed before measurement, with complex preparation processes and being not conducive to the rapid non-destructive detection of samples. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a magnetic anisotropy quantification method and system based on detecting terahertz signals using a rotating magnetic field, which are non-contact, have a simple process, and are non-destructive for detection.

[0005] To achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0006] A magnetic anisotropy quantification method based on detecting terahertz signals using a rotating magnetic field, comprising the following steps:

[0007] (1) Obtain the magnetic / non-magnetic material film to be measured, where the magnetic material film to be measured includes at least one non-magnetic thin film layer with strong spin-charge current conversion and at least one magnetic thin film layer with magnetic anisotropy;

[0008] (2) Emit a femtosecond laser pump pulse to the magnetic / non-magnetic material film to be measured. Generate a spin current through the irradiation of the magnetic thin film layer by the femtosecond laser pump pulse and inject it into the non-magnetic thin film layer. The non-magnetic thin film layer converts the spin current into a transient charge current on the picosecond time scale, thereby radiating terahertz waves;

[0009] (3) Apply a magnetic field to the magnetic / non-magnetic material film to be measured, where the plane of the magnetic field is perpendicular to the incident direction of the femtosecond laser pump pulse;

[0010] (4) Set the relative time delay between the femtosecond laser probe pulse and the femtosecond laser pump pulse to the signal position of the terahertz wave, and continuously rotate the angle of the applied magnetic field. Detect the terahertz wave signals at different magnetic field angles through the femtosecond laser probe pulse;

[0011] (5) Calculate the magnetic anisotropy field of the magnetic thin film layer based on the terahertz wave signals at different magnetic field angles.

[0012] Further, the material of the magnetic thin film layer is any one of magnetic metals Fe, Co, Ni, CoFeB, magnetic oxides Fe2O3, Fe3O4, Cr2O3, NiO, CoO, magnetic alloys Ni2MnGa, Ni2MnIn, Co2MnAl, Co2VGa, two-dimensional magnetic materials CrI3, CrBr3, MnBi2Te4, Fe3GeTe2, CrSBr, and magnetic semiconductor materials (Ga,Mn)As.

[0013] Further, the material of the non-magnetic thin film layer is any one of non-magnetic metals Pt, W, Pd, Ta, interface inversion symmetry breaking Ag / Bi, and topological insulators Bi2Se3, Bi2Te3.

[0014] Further, apply a magnetic field generated by an electromagnet, a superconducting coil, or a permanent magnet material to the magnetic / non-magnetic material film to be measured. Realize the angular rotation of the applied magnetic field relative to the sample by changing the current magnitude in two pairs of orthogonal vector magnet coils, or rotating the electromagnet, the permanent magnet direction, or rotating the magnetic / non-magnetic material film to be measured.

[0015] Further, the femtosecond laser probe pulse detects terahertz signals that are mutually orthogonal, specifically selecting a set of horizontal component P x and vertical component P y for detection.

[0016] Further, step (5) specifically includes:

[0017] (5.1) Obtain the horizontal component P of the terahertz wave signal detected by the femtosecond laser probe pulse at different magnetic field angles x and the vertical component P y ;

[0018] (5.2) Vectorially superimpose the horizontal component P x and the vertical component P y at different magnetic field angles to obtain the vector form of the terahertz wave signal at different magnetic field angles;

[0019] (5.3) Obtain the polarization angle of the terahertz wave at different magnetic field angles. According to the relationship that its electric field polarization angle is always perpendicular to the magnetization angle, obtain the magnetization angle of the magnetic thin film layer at different magnetic field angles:

[0020]

[0021] In the formula, θ M represents the magnetization angle of the magnetic thin film layer;

[0022] (5.4) If the magnetic thin film layer has uniaxial magnetocrystalline anisotropy, substitute it into the following formula to obtain the uniaxial anisotropy field of the magnetic thin film layer:

[0023]

[0024] In the formula, H u represents the magnitude of the uniaxial magnetic anisotropy field of the magnetic thin film layer, H represents the applied magnetic field, and θ H represents the magnetic field angle;

[0025] If the magnetic thin film layer has four-fold magnetocrystalline anisotropy, substitute it into the following formula to obtain the four-fold anisotropy field of the magnetic thin film layer:

[0026]

[0027] In the formula, H4 represents the magnitude of the four-fold magnetic anisotropy field of the magnetic thin film layer.

[0028] A magnetic anisotropy quantitative system for detecting terahertz signals based on a rotating magnetic field, comprising:

[0029] A magnetic / non-magnetic material film to be measured, the magnetic / non-magnetic material film to be measured includes at least one non-magnetic thin film layer with strong spin-charge current conversion and at least one magnetic thin film layer with magnetic anisotropy;

[0030] A magnetic field application module for applying a magnetic field to the magnetic / non-magnetic material film to be measured, and the magnetic field plane is perpendicular to the incident direction of the femtosecond laser pump pulse;

[0031] A magnetic field rotation module, which is used to continuously rotate the angle of the applied magnetic field to obtain a rotating magnetic field. The magnetic field rotation module realizes the rotation of the angle of the applied magnetic field relative to the sample by changing the current magnitudes in two pairs of orthogonal vector magnet coils, or rotating the electromagnet or the permanent magnet direction, or rotating the magnetic / non-magnetic material film to be measured;

[0032] A femtosecond laser pump-probe module, which is used to emit femtosecond laser pump pulses to the magnetic / non-magnetic material film to be measured; spin current is generated by irradiating the magnetic thin film layer with the femtosecond laser pump pulses and injected into the non-magnetic thin film layer, and the non-magnetic thin film layer converts the spin current into a transient charge current on the picosecond time scale, thereby radiating terahertz waves;

[0033] A magnetic anisotropy field calculation module, which is used to calculate the magnetic anisotropy field of the magnetic thin film layer according to the terahertz wave signals at different magnetic field angles.

[0034] Furthermore, the magnetic anisotropy field calculation module specifically includes:

[0035] A data acquisition unit, which is used to acquire the horizontal component P of the terahertz wave signals at different magnetic field angles detected by the femtosecond laser probe pulse x and the vertical component P y ;

[0036] A vector superposition unit, which is used to perform vector superposition on the horizontal component P x and the vertical component P y at different magnetic field angles to obtain the vector form of the terahertz wave signals at different magnetic field angles;

[0037] A magnetization angle calculation unit, which is used to obtain the polarization angle of the terahertz electric field at different magnetic field angles, and according to the relationship that its electric field polarization angle is always perpendicular to the magnetization angle, obtain the magnetization angle of the magnetic thin film layer at different magnetic field angles:

[0038]

[0039] In the formula, θ M represents the magnetization angle of the magnetic thin film layer;

[0040] An anisotropy field calculation unit, which is used to substitute into the following formula to obtain the uniaxial anisotropy field of the magnetic thin film layer if the magnetic thin film layer has uniaxial magnetocrystalline anisotropy:

[0041]

[0042] In the formula, H u represents the magnitude of the uniaxial magnetic anisotropy field of the magnetic thin film layer, H represents the externally applied magnetic field, and θ H represents the magnetic field angle;

[0043] If the material of the magnetic thin film layer has four-fold magnetocrystalline anisotropy, substituting it into the following formula gives the four-fold anisotropy field of the magnetic thin film layer:

[0044]

[0045] In the formula, H4 represents the magnitude of the four-fold magnetic anisotropy field of the magnetic thin film layer.

[0046] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention has the advantages of non-contact measurement, no need for micro-nano processing, etc. The sample preparation process is simple, which is conducive to the rapid and non-destructive detection of samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram of the quantitative method of magnetic anisotropy for detecting terahertz signals through a rotating magnetic field in the present invention;

[0048] Figure 2 For the Co / Pt / MgO(110) sample in the present invention along the hard axis and the easy axis H x / / MgO

[001] two crystal orientations, the orthogonal terahertz time-domain spectroscopic information diagram measured;

[0049] Figure 3 Schematic diagram of the two-dimensional terahertz time-domain electric field of the Co / Pt / MgO(110) sample at different magnetic field angles in the present invention;

[0050] Figure 4 For the Co / Pt / MgO(110) sample along MgO

[001] and The dependence of the orthogonal terahertz peak electric field on the magnetic field measured in two crystal orientations;

[0051] Figure 5 Variation relationship diagram of the two orthogonal terahertz peak electric fields measured for the Co / Pt / MgO(110) sample with the magnetic field angle at different magnetic field magnitudes;

[0052] Figure 6 Dependence of the magnetic field angle on the magnetization angle for the Co / Pt / MgO(110) sample at different magnetic fields;

[0053] Figure 7 Variation relationship diagram of the ratio of the external magnetic field to the uniaxial anisotropy with the magnitude of the external magnetic field. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention.

[0055] Embodiment 1

[0056] Embodiment 1 of the present invention provides a method for quantitatively measuring magnetic anisotropy based on detecting terahertz signals using a rotating magnetic field, as Figure 1 and Figure 2 shown, which includes the following steps:

[0057] (1) Obtain a magnetic / non-magnetic material film to be measured, where the magnetic / non-magnetic material film to be measured includes at least one non-magnetic thin film layer with strong spin-charge current conversion and at least one magnetic thin film layer with magnetic anisotropy.

[0058] Among them, the material of the magnetic thin film layer is any one of magnetic metals Fe, Co, Ni, CoFeB, magnetic oxides Fe2O3, Fe3O4, Cr2O3, NiO, CoO, magnetic alloys Ni2MnGa, Ni2MnIn, Co2MnAl, Co2VGa, two-dimensional magnetic materials CrI3, CrBr3, MnBi2Te4, Fe3GeTe2, CrSBr, and magnetic semiconductor materials (Ga,Mn)As. The material of the non-magnetic thin film layer is a material with strong spin-orbit coupling, specifically any one of non-magnetic metals Pt, W, Pd, Ta, interface inversion symmetry-breaking Ag / Bi, and topological insulators Bi2Se3, Bi2Te3, which can convert spin current into charge current through the inverse spin Hall effect or Rashba-Edelstein effect, and then radiate terahertz waves.

[0059] (2) Emit a femtosecond laser pump pulse to the magnetic / non-magnetic material film to be measured. The femtosecond laser pump pulse irradiates the magnetic thin film layer to generate a spin current, which is injected into the non-magnetic thin film layer. The non-magnetic thin film layer converts the spin current into a transient charge current on the picosecond time scale, thereby radiating terahertz waves.

[0060] (3) Apply a magnetic field to the magnetic / non-magnetic material film to be measured, where the plane of the magnetic field is perpendicular to the incident direction of the femtosecond laser pump pulse.

[0061] Among them, a magnetic field is applied to the magnetic / non-magnetic material film to be measured through an electromagnet, a superconducting coil, or a permanent magnet material. By changing the current magnitudes of two pairs of orthogonal vector magnet coils, or rotating the electromagnet or permanent magnet direction, or rotating the magnetic material film to be measured, the angle of the applied magnetic field relative to the sample is rotated. The magnitude of the applied magnetic field should not be much larger or much smaller than the anisotropy field magnitude.

[0062] (4) Set the relative time delay between the femtosecond laser probe pulse and the femtosecond laser pump pulse to the signal position of the terahertz wave, and continuously rotate the angle of the applied magnetic field. Detect the terahertz wave signal at different magnetic field angles through the femtosecond laser probe pulse.

[0063] Among them, the femtosecond laser probe pulse performs terahertz wave detection in directions perpendicular to each other of the magnetic material film to be measured. Usually, a set of horizontal components P x and vertical components P y are used for detection.

[0064] (5) Calculate the magnetic anisotropy field of the magnetic thin film layer according to the polarization angle of the terahertz wave signal at different magnetic field angles.

[0065] Step (5) specifically includes:

[0066] (5.1) Obtain the horizontal component P x and vertical component P y of the terahertz wave signal detected by the femtosecond laser probe pulse at different magnetic field angles;

[0067] (5.2) Perform vector superposition on the horizontal component P x and vertical component P y at different magnetic field angles to obtain the vector form of the terahertz wave signal at different magnetic field angles;

[0068] (5.3) Obtain the polarization angle of the terahertz wave at different magnetic field angles. According to the relationship that its electric field strength is always perpendicular to the magnetization angle, obtain the magnetization angle of the magnetic thin film layer at different magnetic field angles:

[0069]

[0070] In the formula, θ M represents the magnetization angle of the magnetic thin film layer;

[0071] (5.4) If the magnetic thin film layer has uniaxial magnetocrystalline anisotropy, substitute it into the following formula to obtain the uniaxial anisotropy field of the magnetic thin film layer:

[0072]

[0073] In the formula, represents the magnitude of the uniaxial magnetic anisotropy field of the magnetic thin film layer, H represents the applied magnetic field, θ H represents the magnetic field angle, θ M represents the magnetization angle of the magnetic thin film layer, M represents the magnetization intensity of the sample, and K u represents the uniaxial magnetic anisotropy constant of the sample.

[0074] If the magnetic thin film layer has four-fold magnetocrystalline anisotropy, substitute it into the following formula to obtain the four-fold anisotropy field of the magnetic thin film layer:

[0075]

[0076] In the formula, It represents the magnitude of the fourth-order magnetic anisotropy field of the magnetic thin film layer, and K4 represents the fourth-order magnetic anisotropy constant of the sample.

[0077] The working principle of the present invention is as follows: When rotating the externally applied bias magnetic field, the direction of the effective field generated under the combined action of the anisotropy field and the externally applied magnetic field is not always parallel to the direction of the externally applied magnetic field. By obtaining the relationship between the terahertz signal and the direction of the externally applied magnetic field, the dependence of the effective field on the externally applied magnetic field is obtained, and then the magnitude of the anisotropy field is quantified, specifically as follows:

[0078] (1) Terahertz wave and its relationship with the magnetization direction of the sample:

[0079] The femtosecond laser pump pulse irradiates the magnetic material to generate a spin-polarized charge current, and transfers the carried spin angular momentum to the non-magnetic material in the adjacent layer, which is converted into a transient charge current on the picosecond time scale due to the inverse spin Hall effect or the Rashba-Edelstein effect. The transient charge current will radiate terahertz waves. According to the conversion rule of the inverse spin Hall effect E THz ∝γj s ×σ, or the conversion rule of the Rashba-Edelstein effect E THz ∝λj s ×z, where E THz represents the terahertz wave electric field vector, γ represents the spin Hall angle of the non-magnetic material layer, M represents the magnetization intensity, j s represents the spin current, σ represents the spin direction, and z represents the potential gradient direction perpendicular to the interface. Since the polarization direction of the generated terahertz wave is always perpendicular to the spin direction and the interface potential gradient direction, the magnetization direction of the magnetic thin film layer is obtained by detecting the vector synthesis of the terahertz wave polarization direction.

[0080] (2) Principle of the method for quantitatively measuring the magnetic anisotropy field by detecting terahertz waves using a rotating magnetic field:

[0081] Considering a single-domain coherent rotation system, the magnetization direction of its material is determined by the combined action of the anisotropy field and the Zeeman field.

[0082] For a uniaxial magnetocrystalline anisotropy system, its energy can be described as:

[0083] E = K u sin 2 θ M -HMcos(θ H -θ M ) (1)

[0084] In the formula, E represents the total energy, K u represents the uniaxial magnetocrystalline anisotropy constant of the magnetic thin film layer, θ M represents the magnetization angle of the magnetic thin film layer, H represents the magnetization intensity, H represents the external magnetic field, θH represents the magnetic field strength.

[0085] For a ferromagnetic system, since the time scale of precession is much larger than the sub-picosecond order of magnitude, it is approximately considered that the total energy of the system does not evolve with sub-picosecond time. Considering the magnetic system in an equilibrium state:

[0086]

[0087] Furthermore, we obtain

[0088]

[0089] wherein, H u is the uniaxial anisotropy field, and H u = 2K u / M.

[0090] For a four-fold anisotropic system, its energy can be described as:

[0091] E = K4sin 2 θ M cos 2 θ M - HMcos(θ H - θ M ) (4)

[0092] wherein, K4 represents the four-fold magnetic anisotropy constant of the magnetic thin film layer.

[0093] Similarly, considering the magnetic system in an equilibrium state, we can obtain:

[0094]

[0095] wherein, H4 is the four-fold anisotropy field, and H4 = K4 / M.

[0096] Therefore, by fixing the pump-probe time to the terahertz wave signal position, rotating the magnetic field angle, measuring the vector terahertz electric field at different magnetic field angles, obtaining the polarization direction of the terahertz wave, and then obtaining the dependence relationship between the magnetic field angle and the magnetization angle, the magnitude of the anisotropy field can be obtained according to this dependence relationship and formula (3) or (5).

[0097] Example Two

[0098] Example Two of the present invention provides a magnetic anisotropy quantitative system for detecting terahertz signals based on a rotating magnetic field, which is used to execute the method described in Example One, and includes:

[0099] a magnetic / non-magnetic material film to be measured, where the magnetic / non-magnetic material film to be measured includes at least one non-magnetic thin film layer with strong spin-charge current conversion and at least one magnetic thin film layer with magnetic anisotropy;

[0100] A magnetic field application module for applying a magnetic field to a magnetic / non-magnetic material film to be measured, wherein the plane of the magnetic field is perpendicular to the incident direction of the femtosecond laser pump pulse;

[0101] A magnetic field rotation module for continuously rotating the angle of the applied magnetic field to obtain a rotating magnetic field. The magnetic field application module applies a magnetic field at the position of the magnetic / non-magnetic material film to be measured through an electromagnet, a superconducting coil or a permanent magnet material. The magnetic field rotation module realizes the angular rotation of the applied magnetic field relative to the sample by changing the current magnitude in two pairs of orthogonal vector magnet coils, or rotating the electromagnet or permanent magnet direction, or rotating the magnetic / non-magnetic material film to be measured.

[0102] A femtosecond laser pump-probe module for emitting a femtosecond laser pump pulse to a magnetic / non-magnetic material film to be measured. The femtosecond laser used is compatible with a femtosecond laser oscillator and a femtosecond laser amplifier; the terahertz electric field signal is collected through the electro-optic sampling principle and the balanced detection method, and the terahertz electric field signals at different times are obtained through the pump-probe technology. The applied magnetic field is along a certain direction in the plane of the magnetic material. A spin current is generated by irradiating the magnetic thin film layer with a femtosecond laser pump pulse and injected into the non-magnetic thin film layer. The non-magnetic thin film layer converts the spin current into a transient charge current on the picosecond time scale, thereby radiating terahertz waves; the magnetic field angle is rotated, and the terahertz wave signals at different magnetic field angles are detected by a femtosecond laser detection pulse;

[0103] A magnetic anisotropy field calculation module for calculating the magnetic anisotropy field of the magnetic thin film layer according to the terahertz wave signals at different magnetic field angles.

[0104] Among them, the magnetic anisotropy field calculation module specifically includes:

[0105] A data acquisition unit for acquiring the horizontal component P of the terahertz wave signals at different magnetic field angles detected by the femtosecond laser detection pulse x and the vertical component P y ;

[0106] A vector superposition unit for vectorially superposing the horizontal component P x and the vertical component P y at different magnetic field angles to obtain the vector form of the terahertz wave signals at different magnetic field angles;

[0107] A magnetization angle calculation unit for obtaining the polarization angle of the terahertz electric field at different magnetic field angles, and obtaining the magnetization angle of the magnetic thin film layer at different magnetic field angles according to the relationship that its electric field polarization angle is always perpendicular to the magnetization angle:

[0108]

[0109] Where, θ M represents the magnetization angle of the magnetic thin film layer;

[0110] The anisotropy field calculation unit is used to substitute the following formula to obtain the uniaxial anisotropy field of the magnetic thin film layer if the magnetic thin film layer has uniaxial magnetocrystalline anisotropy:

[0111]

[0112] Where, H u represents the magnitude of the uniaxial magnetic anisotropy field of the magnetic thin film layer, H represents the applied magnetic field, and θ H represents the magnetic field angle;

[0113] If the magnetic thin film layer has four-fold magnetocrystalline anisotropy, substitute the following formula to obtain the four-fold anisotropy field of the magnetic thin film layer:

[0114]

[0115] Where, H4 represents the magnitude of the four-fold magnetic anisotropy field of the magnetic thin film layer.

[0116] The system provided in the second embodiment of the present invention can be used to execute the method provided in the first embodiment of the present invention, and has the corresponding functions and beneficial effects for executing the method.

[0117] It should be noted that in the above embodiments of the determination device, the various units and modules included are only divided according to functional logic, but are not limited to the above division, as long as the corresponding functions can be realized; in addition, the specific names of the functional units are only for the convenience of mutual distinction and do not limit the protection scope of the present invention.

[0118] The above-described embodiments are merely illustrative. The modules described as separate components may or may not be physically separated, and the components shown as modules may or may not be physical modules, that is, they may be located in one place or distributed to multiple network modules. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. Those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus a necessary general hardware platform, and of course, it can also be implemented only by hardware as long as the functions or effects can be achieved.

[0119] Next, the present invention is experimentally verified.

[0120] Figure 1Schematic diagram of the measurement experimental setup. The femtosecond laser pulses generated by the laser emitter used in the experiment have a wavelength of 800 nm and a pulse width of 35 fs. By building a pump-probe terahertz time-domain spectroscopy optical path, the pulsed laser is divided into pump laser and probe laser; the sample (the magnetic / non-magnetic layer heterojunction to be measured) is a Co / Pt bilayer film grown on an MgO(110) substrate, where Co is the magnetic layer and Pt is the non-magnetic layer; after the pump light excites the sample to radiate terahertz signals, the detection direction is fixed along the horizontal (easy axis of the sample) and vertical (hard axis of the sample) directions respectively, and the time delay is fixed at the terahertz peak position. By continuously rotating the in-plane magnetic field angle perpendicular to the incident direction of the pump light, the terahertz peak signals along the easy axis and hard axis directions of the sample are obtained, and then the magnitude of the anisotropy field is quantified through the relationship between the signal and the applied magnetic field angle.

[0121] Figure 2 For the Co / Pt / MgO(110) sample, magnetic fields (H y , H x ) are applied in the hard axis and easy axis directions respectively, and detection (P y , P x ) is performed along the hard axis and easy axis. By changing the time delay between the pump light and the probe light, the terahertz time-domain spectroscopy signal is obtained. For the Co / Pt / MgO(110) sample, the terahertz signals (H y P y ) when the magnetic field is along the hard axis and the detection direction is along the hard axis are basically the same as the terahertz signals (H x P y ) when the magnetic field is along the easy axis and the detection direction is along the hard axis; while when the magnetic field is along the hard axis direction and the detection is along the easy axis direction (H y P x ), a smaller terahertz component is generated. This difference in terahertz signals is exactly the contribution of the magnetocrystalline anisotropy of the sample.

[0122] Figure 3 Terahertz vector electric field time-domain signals measured at several characteristic magnetic field angles. The magnetic field angle θ H is defined as the angle between the applied magnetic field and the easy axis direction of the sample. When the applied magnetic field direction is along the easy axis of the sample, the generated terahertz polarization direction is along the hard axis of the sample; when θ H is 60° and 240°, the generated terahertz polarization direction deviates from the hard axis by about 12°; when θ H is 120° and 300°, the generated terahertz polarization direction deviates from the hard axis by about -12°. At several characteristic magnetic field angles, the generated terahertz polarization directions are basically near the hard axis.

[0123] Figure 4 Shows at H y P x , H y Py , H x P x and H x P y The variation relationship of the terahertz peak electric field with the magnetic field under four configurations. When the external magnetic field is along the easy axis direction, the terahertz signal is emitted along the hard axis direction, and the generated peak terahertz signal shows a square loop variation with the magnetic field; when the external magnetic field is along the hard axis direction, the signal measured along the easy axis direction shows an increasing trend with the magnetic field, and the detection along the hard axis direction shows an inclined square loop.

[0124] Figure 5 Shows the variation relationship of the terahertz peak electric field measured along the hard axis and the easy axis directions with the magnetic field angle under three sets of external magnetic fields. When detecting the terahertz signal along the hard axis direction, the peak electric field shows a "step" variation trend with the magnetic field angle; when detecting the terahertz signal along the easy axis direction, the peak electric field shows a "triangle" variation trend with the magnetic field angle. When θ H is near 90° and 270°, with the increase of the magnetic field, the component signal in the easy axis direction increases, and the signal component in the hard axis direction decreases. When the magnetic field is at a certain angle, the magnetization angle θ M = arctan(P y / P x ), and then the dependence relationship between the magnetic field angle θ H and the magnetization angle θ M under different magnetic fields can be obtained, as shown in Figure 6 .

[0125] By considering the dependence relationship between the magnetic anisotropy field and the magnetization angle, the magnetic field angle and the magnetic field in formula (3), the magnitude of the uniaxial anisotropy field is quantitatively determined. Figure 7 Shows the dependence relationship between 2H / H k and H. The magnitude of the uniaxial anisotropy field of Co / Pt / Mg(110) obtained is 4.4 ± 0.1 kOe.

[0126] It should be understood that the above embodiments and the descriptions in the specification are only the principles, main features and advantages of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the protection scope of the present invention.

Claims

1. A quantitative method for magnetic anisotropy based on detecting terahertz signals using a rotating magnetic field, characterized in that, It includes the following steps: (1) Obtain the magnetic / non-magnetic material film to be measured, where the magnetic / non-magnetic material film to be measured includes at least one non-magnetic thin film layer with strong spin-charge current conversion and at least one magnetic thin film layer with magnetic anisotropy; (2) Emit a femtosecond laser pump pulse to the magnetic / non-magnetic material film to be measured. Generate a spin current through the irradiation of the magnetic thin film layer by the femtosecond laser pump pulse and inject it into the non-magnetic thin film layer. The non-magnetic thin film layer converts the spin current into a transient charge current on the picosecond time scale, thereby radiating terahertz waves; (3) Apply a magnetic field to the magnetic / non-magnetic material film to be measured, and the plane of the magnetic field is perpendicular to the incident direction of the femtosecond laser pump pulse; (4) Set the relative time delay between the femtosecond laser probe pulse and the femtosecond laser pump pulse to the signal position of the terahertz wave, and continuously rotate the angle of the applied magnetic field. Detect the terahertz wave signal at different magnetic field angles through the femtosecond laser probe pulse; (5) Calculate the magnetic anisotropy field of the magnetic thin film layer based on the terahertz wave signals at different magnetic field angles.

2. A method for quantitatively determining magnetic anisotropy for detecting terahertz signals based on a rotating magnetic field according to claim 1, characterized in that: The material of the magnetic thin film layer is any one of magnetic metals Fe, Co, Ni, CoFeB, magnetic oxides Fe2O3, Fe3O4, Cr2O3, NiO, CoO, magnetic alloys Ni2MnGa, Ni2MnIn, Co2MnAl, Co2VGa, two-dimensional magnetic materials CrI3, CrBr3, MnBi2Te4, Fe3GeTe2, CrSBr, and magnetic semiconductor materials (Ga,Mn)As.

3. A method for quantitatively determining magnetic anisotropy for detecting terahertz signals based on a rotating magnetic field according to claim 1, wherein: The material of the non-magnetic thin film layer is any one of non-magnetic metals Pt, W, Pd, Ta, interface inversion symmetry-breaking Ag / Bi, topological insulators Bi2Se3, Bi2Te3.

4. A method for quantitatively measuring magnetic anisotropy for detecting terahertz signals based on a rotating magnetic field according to claim 1, characterized in that: Apply a magnetic field to the magnetic / non-magnetic material film to be measured, and apply a magnetic field at the position of the magnetic / non-magnetic material film to be measured through an electromagnet, a superconducting coil, or a permanent magnet material.

5. A method for quantitatively determining magnetic anisotropy for detecting terahertz signals based on a rotating magnetic field according to claim 1, characterized in that: Apply a magnetic field generated by an electromagnet, a superconducting coil, or a permanent magnet material to the magnetic / non-magnetic material film to be measured. Realize the angular rotation of the applied magnetic field relative to the sample by changing the current magnitude in two pairs of orthogonal vector magnet coils, or rotating the electromagnet or permanent magnet direction, or rotating the magnetic / non-magnetic material film to be measured.

6. A method for quantitatively determining magnetic anisotropy of detecting terahertz signals based on a rotating magnetic field according to claim 1, characterized in that: The femtosecond laser probe pulse probes the terahertz signals that are orthogonal to each other, and specifically selects a set of horizontal components P x and vertical components P y for detection.

7. A method for quantitatively determining magnetic anisotropy for detecting terahertz signals based on a rotating magnetic field according to claim 1, characterized in that: Step (5) specifically includes: (5.1) Obtain the horizontal component P x and the vertical component P y ; (5.2) Vectorially superimpose the horizontal component P x and the vertical component P y to obtain the vector form of the terahertz wave signal at different magnetic field angles; (5.3) Obtain the polarization angle of the terahertz wave at different magnetic field angles. According to the relationship that its electric field polarization angle is always perpendicular to the magnetization angle, obtain the magnetization angle of the magnetic thin film layer at different magnetic field angles: where θ M represents the magnetization angle of the magnetic thin film layer; (5.4) If the magnetic thin film layer has uniaxial magnetocrystalline anisotropy, substitute it into the following formula to obtain the uniaxial anisotropy field of the magnetic thin film layer: Where, H u represents the magnitude of the uniaxial magnetic anisotropy field of the magnetic thin film layer, H represents the applied magnetic field, and θ H represents the magnetic field angle; If the magnetic thin film layer has four-fold magnetocrystalline anisotropy, substitute it into the following formula to obtain the four-fold anisotropy field of the magnetic thin film layer: In the formula, H4 represents the magnitude of the four-fold magnetic anisotropy field of the magnetic thin film layer.

8. A magnetic anisotropy quantification system for detecting terahertz signals based on a rotating magnetic field, characterized in that It includes: A magnetic / non-magnetic material film to be measured, where the magnetic / non-magnetic material film to be measured includes at least one non-magnetic thin film layer with strong spin-charge current conversion and at least one magnetic thin film layer with magnetic anisotropy; A magnetic field application module for applying a magnetic field to a magnetic / non-magnetic material film to be measured, wherein the plane of the magnetic field is perpendicular to the incident direction of the femtosecond laser pump pulse; A magnetic field rotation module for continuously rotating the angle of the applied magnetic field to obtain a rotating magnetic field; A femtosecond laser pump-probe module for emitting a femtosecond laser pump pulse to the magnetic / non-magnetic material film to be measured; A spin current is generated by irradiating the magnetic thin film layer with the femtosecond laser pump pulse and injected into the non-magnetic thin film layer. The non-magnetic thin film layer converts the spin current into a transient charge current on the picosecond time scale, thereby radiating terahertz waves; A magnetic anisotropy field calculation module for calculating the magnetic anisotropy field of the magnetic thin film layer according to the terahertz wave signals at different magnetic field angles.

9. The magnetic anisotropy quantification system for detecting terahertz signals based on a rotating magnetic field according to claim 8, wherein: The magnetic anisotropy field calculation module specifically includes: A data acquisition unit for acquiring the horizontal component P of the terahertz wave signal at different magnetic field angles detected by a femtosecond laser probe pulse x and the vertical component P y ; A vector superposition unit for vectorially superposing the horizontal component P x and the vertical component P y to obtain the vector form of the terahertz wave signal at different magnetic field angles; A magnetization angle calculation unit for obtaining the polarization angle of the terahertz electric field at different magnetic field angles, and obtaining the magnetization angle of the magnetic thin film layer at different magnetic field angles according to the relationship that its electric field polarization angle is always perpendicular to the magnetization angle; where θ M represents the magnetization angle of the magnetic thin film layer; An anisotropy field calculation unit for substituting into the following formula to obtain the uniaxial anisotropy field of the magnetic thin film layer if the magnetic thin film layer has uniaxial magnetocrystalline anisotropy: Where, H u represents the magnitude of the uniaxial magnetic anisotropy field of the magnetic thin film layer, H represents the externally applied magnetic field, and θ H represents the magnetic field angle; If the material of the magnetic thin film layer has four-fold magnetocrystalline anisotropy, substitute into the following formula to obtain the four-fold anisotropy field of the magnetic thin film layer: In the formula, H4 represents the magnitude of the four-fold magnetic anisotropy field of the magnetic thin film layer.

10. A magnetic anisotropy quantification system for detecting terahertz signals based on a rotating magnetic field according to claim 8, characterized in that: The magnetic field rotation module realizes the angular rotation of the applied magnetic field relative to the sample by changing the current magnitudes in two pairs of orthogonal vector magnet coils, or rotating the direction of the electromagnet or permanent magnet, or rotating the magnetic / non-magnetic material film to be measured.

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