Spin Transfer Torque Induced Magnetization Dynamic In Magnetic Nanostructures

Giant magnetoresistance (GMR) effect had been extensively used in magnetic memory, magnetic sensor and so on. The spin transfer torque was theoretically predicted by Berger and Slonczewski in1996, which was considered as milestone after GMR in magentics. The magnetization can be manipulated by the spin transfer torque rather than the external magnetic field in nanoscale magnetic device. Spin transfer torque holds great promise in the applications of the nonvolatile magnetic random access memory (MRAM), the high-frequency microwave oscillators and the racetrack memory. In this thesis, it is investigated that the spin transfer torque induces magnetization switching under microwave magnetic field assisting by micromagnetic simulation, and the Seebeck voltage is systematically investigated in lateral spin valve by two different measurement techniques. The main results of this thesis are as following:(一)Spin transfer torque investigated by micromagneticIn this section, the spin transfer torque driven magnetization reversal in spin valve or magnetic tunnel junction (MTJ) is studied based on the extense Landau-Lifshitz-Gilbert equation by micromagnetic simulation.(1) The critical current density of magnetization switching induced STT in spin valve and magnetic tunnel junction is systematically investigated under microwave magnetic field assisting by micromagnentic simulation. The simulation results indicate that critical current density and magnetization switching time can be obviously reduced due to the introduction of microwave magnetic field. And the frequency and amplitude of microwave magnetic field can strongly affect both critical current density and magnetization switching time. Critical current density and magnetization switching time decrease with increasing microwave amplitude, when microwave frequency is fixed. The minimums of critical current density and magnetization switching time can be obtained, when microwave frequency comes up to the optimal frequency, which is the natural ferromagnetic resonance frequency of spin valve and MTJ. At one time, the mechanism of magnetization switching changes from nucleation reversal to coherent rotation.(2) It is investigated that the STT inducing magnetization reversal is assisted by three types microwave magnetic field (circularly polarized, in-plane linearly polarized and out-plane linearly polarized microwave magnetic field). The reducation of critical current density and magnetization switching time under circularly polarized microwave assisted is bigger than that under linearly polarized microwave assisted.(3) Perpendicular anisotropy is very important, because it has an impact not only on dynamic magnetic, but also on STT inducing magnetization reversal. Micromagneitc simulation is carried out to build up a direct relationship between critical current density, natural resonance frequency and uniaxial magnetic anisotropy constant in In-plane spin valve and Out-plane MTJ. In in-plane spin valve, the critical current densiy and natural resonance frequency linearly decrease with the increasing uniaxial magnetic anisotropy constant. But the critical current densiy linearly increases with increasing uniaxial magnetic anisotropy constant in out-plane MTJ.(4) Dynamic magetic susceptibilities and static magnentization configurations of permally antidot array with two dimensional periodic boundary condition (2DPBC) are investigated by micromagnetic simulation. The stripy-shaped domains appear in antidot array through the holes, which is originated from the shape anisotropy and magnetic dipolar interaction of neighboring repeating element. The natural resonance frequency can be adjusted by changing the inter-holes distance, inner radius and thinkness of antidot array film. The natural resonance frequency weakly varies with thinkness.(二)Seebeck effect in lateral FM/NM hybrid nanstuctureIn this section, the Seebeck effect is systematically investigated in three type lateral Ferromagnetic/Nonmagnetic (FM/NM) hybrid nanostructures. The Seebeck voltage in lateral FM/NM hybrid nanostructure can be obtained by the measurment of different resistance using ac lock-in technology or DC current measurement method. The bias-current dependence background voltage in the conventional nonlocal spin valve measurement is reasonably explained by the Seebeck voltage induced by Peltier effect and Joule heating. The Seebeck voltage curve as function of DC bias current, which is only induced by Joule heating, is symmetry as parabolic function of DC bias current at IDC=0mA axis. The origin of the Seebeck voltage asymmetry curve is the Peltier effect in lateral FM/NM hybrid. The temperature difference between two probes is about2K, when DC bias current is1.5mA.12%of the total thermal power is dispersed through the SiO2/Si substrate via the Cu wire. And there is not Seebeck effect in the same material.