| The vortex state composed of the magnetic moment of the crimp is a basic equilibrium state of the soft magnetic material,which is widely present in the nanomaterial of square or circular shape,and has abundant physical phenomena.In 2000,T.Shinjo first observed the existence of the vortex state in the magnetic nano-point by the magnetic microscope,causing great interest in the academic community.The discovery of the STT effect provides a new method for manipulating the nanoscale magnetic solitons.So that the application of the vortex in the fields of a novel non-volatile memory of a current-induced switch and a direct-current-driven microwave oscillator is realized.The switching mode induced by the polarization current replaces the traditional field-induced switch,so that the power consumption of the device is greatly reduced,and the integration level of the product is improved.The research object of the vortex state is mainly focused on the in-plane anisotropic material,and the perpendicular magnetic anisotropy MTJ has the advantages of low switching current density,high thermal stability,and gradually becomes the main flow storage unit of the STT-MRAM,compared with the traditional surface-in-plane magnetic anisotropy tunnel junction.The vertical magnetic anisotropy is formed by the equilibrium demagnetization energy,the single-axis vertical magnetic anisotropy energy,the exchange coupling energy and the Zeeman energy generated by the polarization layer stray field.Therefore,it is much more complicated than the magnetic anisotropy in the surface,and it is difficult to use the analytical method for analysis.In view of the above problems,this paper studies the physical mechanism and the correlation with the key parameters in the vertical magnetic anisotropic MTJ by the method of micro-magnetic simulation,and has obtained the following research results:(1)Using the micro-magnetic simulation tool OOMMF to create a vertical magnetic anisotropy MTJ model,the electrical properties of the vertical magnetic anisotropy MTJ and the surface-in-plane magnetic anisotropic MTJ are compared.the vertical magnetic anisotropy mtj has a shorter switching time at the same switching current density as compared to the tunnel junction in the face.Meanwhile,in the vertical magnetic anisotropic MTJ,the time difference between the state of the P state and the state of the AP and the state of the AP to the P state is small,and the uniformity of the switching between different states is higher than that of the magnetic anisotropy MTJ in the plane.In the process of the switch,the magnetization oscillation amplitude of the vertical magnetic anisotropy MTJ is small and gentle,and the reliability is higher than the in-plane magnetic anisotropy MTJ.(2)The generation and stabilization process of the free layer vortex distribution state of the vertical magnetic anisotropic MTJ is studied by the micro-magnetic simulation,and the time evolution rule of the magnetization profile and the precession track of the magnetic moment in the specific position are analyzed.The results show that,when the vertical magnetic anisotropy constant is less than a certain critical value,the formation of the vortex state in the vertical magnetic anisotropy MTJ can be obtained by the natural relaxation of the magnetization state which is uniformly distributed in the plane of the initial vertical magnetic disk.The relaxation process of the magnetization can be divided into three parts: nucleation,periodic precession and ringing.During the formation of the vortex,there is a great difference in the precession amplitude of the magnetic moment at different positions of the magnetic disk,which is gradually reduced from the edge of the disk to the central position,but the precession process and time synchronization of the magnetic moment at different positions are synchronized.By analyzing the time dependence of the internal energy variation of the system during the relaxation process,it is revealed that the formation of the vortex state is the result of the magnetic field energy,the exchange energy and the vertical magnetic anisotropy energy balance.(3)The correlation between the generation process of the vortex state and the vertical magnetic anisotropy constant in the vertical magnetic anisotropic MTJ is simulated.the generation time of the vortex state and the vertical magnetic anisotropy constant are not strictly monotone relation,and three extreme values are generated.By analyzing the time evolution of the magnetization distribution in different vertical magnetic anisotropy constants,two different generation modes of the vortex are found: the nucleation-cycle precession-damping oscillation and the nucleation-damping oscillation.The correlation between the magnetic moment angle and the vertical magnetic anisotropy constant of different precession processes is studied by extracting the precession track diagram of the magnetic moment in the specific position of the magnetic disk.The physical mechanism of non-monotonic change of the vortex generation time and the vertical magnetic anisotropy constant is revealed in combination with the change of the internal energy.(4)The influence of the spin-polarized current on the process of uniform distribution of the initial magnetization(consistent with the plane of the perpendicular magnetic disk)was simulated.The relationship between the inversion process and the vertical magnetic anisotropy constant is analyzed.The results show that,under different vertical magnetic anisotropy constants,the final stable state of the magnetization distribution of the free layer can be divided into three different results: the vortex state,the uniform distribution state and the original state.the different magnetization distribution states can be attributed to a trade-off between demagnetization,exchange and vertical magnetic anisotropy energy.The generation of the vortex state is also related to the non-uniform stray field produced by the polarizing layer,and the final stable magnetization size depends on the vertical magnetic anisotropy constant.The vortex and the uniform distribution state have different turn-over processes,and the vortex states are: the nucleation-cycle precession-damping oscillation;the uniform distribution is: the nucleation-periodic precession-spin-wave excitation.The increase of the turn-over time with the increase of the vertical magnetic anisotropy constant is mainly attributed to the non-two different turn-over processes.(5)The dynamic process of the polarization current induced by the polarization current in the vertical magnetic anisotropic MTJ is simulated,and the influence of the polarization current density and the vertical magnetic anisotropy constant on the vortex polarization turnover time and the vortex hand property is analyzed.Under the condition of the same current density,the polarization reversal time exhibits a tendency to increase in advance with the increase of the vertical magnetic anisotropy constant.It is mainly attributed to the different modes of the vortex state polarization reversal under different vertical magnetic anisotropy constants.When the vertical magnetic anisotropy constant is small,the different polarization turning modes according to the polarization current are divided into two types: the disappearance of the vortex core precession vortex core the magnetization reversal reverse vortex core generating reverse vortex core precession;Vortex core precession vortex core polarization-inversion-spin-wave excitation-spin-wave excitation-precession of the counter-vortex core.When the vertical magnetic anisotropy constant is large,the polarization reversal mode is: the reduction of the vortex core-the polarization reversal of the vortex core-the generation of the spin-wave excitation-counter-vortex core.In this mode,the vortex core polarization reversal process does not produce precession. |
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