CN113126010B - Method for measuring spin polarizability of magnetic material - Google Patents

Method for measuring spin polarizability of magnetic material Download PDF

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CN113126010B
CN113126010B CN202110295087.XA CN202110295087A CN113126010B CN 113126010 B CN113126010 B CN 113126010B CN 202110295087 A CN202110295087 A CN 202110295087A CN 113126010 B CN113126010 B CN 113126010B
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车仁超
张瑞轩
裴科
王龙圆
张捷
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Fudan University
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Abstract

The invention relates to a method for measuring the spin polarizability of a magnetic material, which comprises the steps of firstly measuring the magnetic material by using an in-situ Lorentz transmission electron microscope test technologyPulse current IexpActual moving speed v of magnetic domain wall under actionexpThen, the magnetic domain wall I of the same magnetic material under the action of the complete spin polarization current is calculated by utilizing the micro-magnetic simulationtheLower theoretical speed of motion vtheAnd then the spin polarization rate P of the magnetic material is calculated through data comparison between the actual data and the theoretical data. Compared with the traditional spin polarizability testing method utilizing synchrotron radiation and magnetic circular dichroism, the method is simpler and more convenient, and provides a more convenient technical method for spin polarizability analysis of a novel magnetic material in the future.

Description

Method for measuring spin polarizability of magnetic material
Technical Field
The invention belongs to the technical field of material parameter measurement, and relates to a method for measuring the spin polarizability of a magnetic material.
Background
In recent years, with the rapid development of spintronics, studies on the electrical transport characteristics of magnetic materials have been made in depth. Especially, the development of magnetic memory technology is continuous, so that the memory devices related to the spin transfer torque effect and the spin orbit torque effect have become the main direction of future development. Corresponding novel magnetic materials, such as magnetic sigramic materials, two-dimensional magnetic materials and the like, are put into device exploration in many ways. In many experiments, the spin polarizability of a magnetic material is a very important parameter that determines the degree of spin polarization of a current flowing through the material, as well as the magnitude of the generated spin torque. However, the technical method for obtaining the spin polarizability of the material is still very lacking.
The theoretical spin polarizability of a magnetic material can be estimated by analyzing the energy band structure near the fermi surface through the calculation of a first principle, but the theoretical spin polarizability is limited by the calculation force and the method of theoretical calculation, only the theoretical value of the material is generally obtained when the absolute zero degree is approached, and the guidance effect on the actual experiment is difficult to generate. In the experimental method, the mainstream technology adopted at present is to irradiate a material with left-handed/right-handed circularly polarized light generated by synchrotron radiation, resolve the spin magnetic moment and the orbital magnetic moment of the material by utilizing the dichroism of the magnetic circle of the material, and further analyze and obtain the spin polarizability of the material. However, the synchrotron radiation experiment is complex, the cost is high, resources are in short supply, and the synchrotron radiation experiment is difficult to popularize in a common laboratory, which seriously hinders the exploration progress of the novel magnetic material. Therefore, it is desirable to provide a simpler method for measuring the spin polarizability of a material.
Disclosure of Invention
The invention aims to provide a method for measuring the spin polarizability of a magnetic material, so as to obtain the spin polarizability of the magnetic material easily and quickly.
The purpose of the invention can be realized by the following technical scheme:
the theoretical basis of the invention is the spin transfer torque effect, namely after electrons flow into a magnetic material, the angular momentum of free electrons can be changed under the influence of the spin polarization environment in the material, so that the current becomes spin polarization current; in the process of angular momentum transfer, if there is a region in which spin distribution is not uniform, such as a magnetic domain wall, inside the material, angular momentum transfer torque is continuously accumulated in the region, thereby pushing the region (magnetic domain wall) to move in the direction of electron motion. During this motion, the product of the spin polarizability of the material and the current density of the injected material exhibits a linear relationship with the motion velocity of the domain wall. If the motion speed and current density of a magnetic domain wall of a magnetic material can be measured through experiments, the theoretical motion speed of the magnetic domain wall under the action of a fully polarized high-density current is calculated through micro-magnetic simulation, and the actual spin polarizability of the material can be calculated.
Specifically, the invention provides a method for measuring the spin polarizability of a magnetic material, which comprises the steps of firstly measuring the pulse current I of the magnetic material by using an in-situ Lorentz transmission electron microscope test technologyexpActual moving speed v of magnetic domain wall under actionexpThen, the magnetic domain wall I of the same magnetic material under the action of the complete spin polarization current is calculated by utilizing the micro-magnetic simulationtheLower theoretical speed of motion vtheThen the self-identity of the magnetic material is calculated by comparing the data between the actual data and the theoretical dataThe spin polarizability P.
Further, the spin polarizability P is calculated by the formula:
Figure BDA0002984033630000021
further, the actual moving velocity v of the domain wallexpThe test process specifically comprises the following steps:
(A) processing a magnetic material into a rectangular structure, and fixing the rectangular structure on an in-situ electrical test chip, so that the left side and the right side of the factory-direction structure are respectively connected with electrodes of the in-situ electrical test chip to obtain a sample;
(B) inserting the sample into a Lorentz transmission electron microscope, and observing a magnetic domain structure in the sample by using a Fresnel mode;
(C) continuously introducing pulse current I to the sample by using an external pulse sourceexpAt the same time, recording the position of the moving magnetic domain structure in the sample, i.e. obtaining the actual moving speed v of the magnetic domain wallexp。
Further, in the step (a), the aspect ratio of the rectangular structure is 3:1 or more.
Further, in the step (A), the thickness of the rectangular structure is less than 150 nm.
Further, in the step (a), the rectangular structure is formed by processing a magnetic material by a focused ion beam processing technique.
Further, in the step (C), the current I is pulsedexpCurrent density of more than 1 x 109A/m2And the pulse width is between 0.5-100 ns.
Further, a theoretical movement velocity v is obtainedtheThe process specifically comprises the following steps:
(a) constructing theoretical models with the same size by using micro-magnetic simulation software according to the three-dimensional shape of an actual magnetic material sample, and dividing the theoretical models into grids, wherein the size of each divided grid is smaller than the magnetostatic interaction length l of the magnetic materialex；
(b) Inputting material parameters which are consistent with the actually measured magnetic material, so that a simulation system based on a theoretical model is freely relaxed to a stable state;
(c) introducing complete spin polarization current I to the stabilized simulation systemtheAnd outputting a frame of magnetic moment distribution at certain intervals to obtain the position change of the magnetic domain structure, so as to calculate the theoretical motion speed v of the magnetic domain wallthe。
Further, the magnetostatic interaction length lexThe calculation formula of (2) is as follows:
Figure BDA0002984033630000031
wherein A is the exchange coefficient, mu0Is a vacuum permeability, MsThe saturation magnetization.
Further, the material parameter inputted in the step (b) includes saturation magnetization MsMagnetocrystalline anisotropy direction, magnetocrystalline anisotropy coefficient and exchange coefficient a.
Compared with the traditional method of utilizing synchronous radiation testing, the invention skillfully utilizes the physical phenomenon that spin polarized current pushes the magnetic domain wall to move, combines practical tests and theoretical calculation, and utilizes a simpler technical method to obtain the spin polarizability. The research process is simplified, and convenience is provided for parameter measurement and experimental design of the novel magnetic material.
Drawings
FIG. 1 shows the movement of a domain wall under a pulse current in an actual experiment;
FIG. 2 is a diagram showing the movement of a domain wall under the action of a spin-polarized current in a theoretical simulation.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.
In the following embodiments or examples, the in situ test chip is from a published patent (patent No.: CN 201811448585.8). The remainder, unless specifically stated otherwise, are intended to indicate conventional commercially available materials in the art, conventional components or conventional processing techniques for performing the corresponding functions.
The invention provides a method for measuring the spin polarizability of a magnetic material, which comprises the steps of firstly measuring the pulse current I of the magnetic material by using an in-situ Lorentz transmission electron microscope test technologyexpActual moving speed v of magnetic domain wall under actionexpThen, the magnetic domain wall I of the same magnetic material under the action of the complete spin polarization current is calculated by utilizing the micro-magnetic simulationtheLower theoretical speed of motion vtheAnd then the spin polarization rate P of the magnetic material is calculated through data comparison between the actual data and the theoretical data.
In some embodiments, the spin polarizability P is calculated by the formula:
Figure BDA0002984033630000041
in some embodiments, the actual motion velocity v of the domain wallexpThe test process specifically comprises the following steps:
(A) processing a magnetic material into a rectangular structure, and fixing the rectangular structure on an in-situ electrical test chip, so that the left side and the right side of the factory-direction structure are respectively connected with electrodes of the in-situ electrical test chip to obtain a sample;
(B) inserting the sample into a Lorentz transmission electron microscope, and observing a magnetic domain structure in the sample by using a Fresnel mode;
(C) continuously introducing pulse current I to the sample by using an external pulse sourceexpAt the same time, recording the position of the moving magnetic domain structure in the sample, i.e. obtaining the actual moving speed v of the magnetic domain wallexp。
Further, in the step (a), the aspect ratio of the rectangular structure is 3:1 or more.
Further, in the step (A), the thickness of the rectangular structure is less than 150 nm.
Further, in the step (a), the rectangular structure is formed by processing a magnetic material by a focused ion beam processing technique.
In some embodiments, in step (C), the current I is pulsedexpCurrent density of more than 1 x 109A/m2And the pulse width is between 0.5-100 ns.
In some embodiments, a theoretical movement velocity v is obtainedtheThe process specifically comprises the following steps:
(a) constructing theoretical models with the same size by using micro-magnetic simulation software according to the three-dimensional shape of an actual magnetic material sample, and dividing the theoretical models into grids, wherein the size of each divided grid is smaller than the magnetostatic interaction length l of the magnetic materialex；
(b) Inquiring and inputting material parameters which are consistent with the actually measured magnetic material, so that a simulation system based on a theoretical model is freely relaxed to a stable state;
(c) introducing complete spin polarization current I to the stabilized simulation systemtheAnd outputting a frame of magnetic moment distribution at certain intervals to obtain the position change condition of the magnetic domain structure, thereby calculating the theoretical motion speed v of the magnetic domain wallthe。
Further, the magnetostatic interaction length lexThe calculation formula of (2) is as follows:
Figure BDA0002984033630000051
wherein A is the exchange coefficient, mu0For vacuum permeability, MsThe saturation magnetization. If the mesh size is larger than the magnetostatic interaction length, the model calculation cannot be converged.
Further, the material parameters inputted in step (b) include saturation magnetization, magnetocrystalline anisotropy direction, magnetocrystalline anisotropy coefficient and exchange interaction coefficient.
The above embodiments may be implemented individually, or in any combination of two or more.
The above embodiments will be described in more detail with reference to specific examples.
Example 1:
first, using a focused ion beam to select Fe material3GeTe2Processing was carried out to obtain a sample of dimensions 12 μm × 4 μm × 0.1 μm, on both sides of which a current path was formed with the chip by depositing Pt.
And secondly, inserting the sample into a Lorentz transmission electron microscope, controlling the temperature of the sample to be 100K, and observing the magnetic domain structure of the sample by utilizing a Fresnel mode.
Thirdly, the current pulse source and the sample are switched on, and the current density is applied at the frequency of 1Hz and is 5.3 multiplied by 1010A/m2Current pulse I with a pulse length of 80ns1By observing and recording the moving position of the fringe magnetic domain in situ, the velocity v of the magnetic domain motion can be obtained1＝0.76m/s。
Fourthly, constructing a simulation system, dividing the size of a grid into 2nm and adopting Fe3GeTe2The material parameters of the material are specifically as follows: the exchange coefficient A is 5X 10-12J/M, saturation magnetization MsIs 3.5 multiplied by 105A/m, Gilbert damping coefficient is 0.01, and magnetocrystalline anisotropy is 6.52 multiplied by 105J/m3The magnetocrystalline anisotropy direction is (0,1, 0). Intensity of applied current I2Is 2 x 1011A/m2Outputting a magnetic moment distribution diagram every 0.1ns, recording the position of the magnetic domain, and calculating the motion speed v of the magnetic domain wall according to the change of the position of the magnetic domain2＝8m/s。
And fifthly, calculating the spin polarizability of the actual current according to a formula:
Figure BDA0002984033630000052
therefore, in general, the method utilizes the basic principle that the spin transfer torque effect linearly correlates the driving speed of the magnetic domain wall in the magnetic material with the spin polarization current density, and measures the movement speed of the magnetic domain wall under the action of the rated current density in an experiment through the in-situ Lorentz transmission electron microscope technology. And then, calculating the theoretical movement speed of the magnetic material under the action of the complete spin polarization current by a micro-magnetic simulation calculation method based on the LLG equation. The actual spin polarizability of the material can be calculated by comparing the theoretical speed with the actual speed. Compared with the traditional spin polarizability testing method utilizing synchrotron radiation and circular dichroism, the method is simpler and more convenient, and provides a more convenient technical method for spin polarizability analysis of a novel future magnetic material.
The embodiments described above are described to facilitate an understanding and use of the invention by those skilled in the art. It will be readily apparent to those skilled in the art that various modifications to these embodiments may be made, and the generic principles described herein may be applied to other embodiments without the use of the inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications within the scope of the present invention based on the disclosure of the present invention.

Claims (7)

1. A method for measuring the spin polarizability of a magnetic material is characterized in that the in-situ Lorentz transmission electron microscope test technology is firstly utilized to measure the pulse current of the magnetic materialI exp Actual speed of motion of domain wall under influencev exp And calculating the magnetic domain wall of the same magnetic material in the complete spin polarization current by using micro-magnetic simulationI the Theoretical speed of motion under actionv the And then the spin polarizability of the magnetic material is calculated by comparing the data between the actual data and the theoretical dataP；
Spin polarizabilityPThe calculation formula of (2) is as follows:
Figure 518429DEST_PATH_IMAGE001
；
actual speed of motion of magnetic domain wallv exp The test process specifically comprises the following steps:
(A) processing a magnetic material into a rectangular structure, and fixing the rectangular structure on the in-situ electrical test chip, so that the left side and the right side of the factory-direction structure are respectively connected with electrodes of the in-situ electrical test chip to obtain a sample;
(B) inserting the sample into a Lorentz transmission electron microscope, and observing a magnetic domain structure in the sample by using a Fresnel mode;
(C) continuously introducing pulse current to the sample by using an external pulse sourceI exp And simultaneously, recording the moving position of the magnetic domain structure in the sample to obtain the actual moving speed of the magnetic domain wallv exp ；
Obtaining theoretical movement speedv the The process specifically comprises the following steps:
(a) according to the three-dimensional shape of the actual magnetic material sample, a theoretical model with the same size is constructed by using micro-magnetic simulation software, and the theoretical model is divided into grids, wherein the size of the divided grids is smaller than the magnetostatic interaction length of the magnetic materiall ex ；
(b) Inputting material parameters which are consistent with the actually measured magnetic material, so that a simulation system based on a theoretical model is freely relaxed to a stable state;
(c) introducing complete spin polarization current to the stabilized simulation systemI the And outputting a frame of magnetic moment distribution at certain intervals to obtain the position change condition of the magnetic domain structure, thereby calculating the theoretical motion speed of the magnetic domain wallv the 。
2. The method according to claim 1, wherein in step (a), the aspect ratio of the rectangular structure is 3:1 or more.
3. A method for measuring the spin polarizability of a magnetic material as claimed in claim 1 wherein in step (a), the rectangular structure has a thickness less than 150 nm.
4. The method according to claim 1, wherein in step (a), the rectangular structure is formed from the magnetic material by focused ion beam processing.
5. A method for measuring the spin polarizability of a magnetic material as claimed in claim 1 wherein in step (C), the pulsed current is appliedI exp Current density of more than 1 x 109 A/m2And the pulse width is between 0.5-100 ns.
6. A method for measuring the spin polarizability of a magnetic material as claimed in claim 1 wherein the magnetostatic interaction length isl ex The calculation formula of (2) is as follows:
Figure 169990DEST_PATH_IMAGE002
wherein, in the step (A),Ain order to exchange the coefficients of action,μ 0 in order to achieve a magnetic permeability in a vacuum,M s the saturation magnetization.
7. A method for measuring the spin polarizability of a magnetic material as claimed in claim 1 wherein the material parameters input in step (b) include saturation magnetization, magnetocrystalline anisotropy direction, magnetocrystalline anisotropy coefficient and exchange interaction coefficient.
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