Micromagnetic Simulation Of Magnetic Vortex-based Spin Torque Nano-oscillator

Since, the giant magnetoresistance(GMR) effect was discovered by Albert Fert and Peter Grunberg, spintronics based on spin-polarized electron transport and the development of related elements had been formed, meanwhile magnetic sensor and data storage technology got rapid development. Subsequently, the spin-transfer-torque(STT) effect was predicted in theory, which was considered as another milestone after GMR effect. Specifically, that is, when spin-polarized current get through the magnetic multilayer film, its polarization electronic carries angular momentum transferred to the local magnetic moment of magnetic layer, and then produce a moment for the local magnetic moment. In the spin valve or tunnel junction structure, STT can stimulate the local magnetic moment, and then occurre different dynamical processing, such as magnetic domain motion, magnetization reversal or stable precession, etc, and then can be applied in microwave oscillators, magnetic sensors, and racetrack memory, ect. In this thesis, we studied magnetic vortex-based spin torque nano-oscillator using micromagnetic simulation, which includes three parts and the main results are as following:1. Magnetic vortex in Dzyaloshinskii-Moriya interactions(1) The vortex state is closely related to the parameter D of Dzyaloshinskii-Moriya interactions(DMI). When the D value reaches a threshold Dthre, the magnetic configuration converts from a vortex state to a helical stripe state. When |D| > Dthre, magnetic configuration is a stripe state, while |D| < Dthre for a vortex state. For D > 0, the size of the vortex core increases as D increases, while the size decreases for that of D < 0.(2) When D = 3 mJ/m2, the oscillation frequency of the vortex is about 3 times higher than that of without DMI, it can support a higher current density and wider current range, and the linewidth is more narrow than when the radius of nanodisk is 50 nm.(3) For D = 3 mJ/m2, the oscillation frequency is higher, current adjust range is wider and the current density is also higher for the smaller sizes of nanodisk.2. The effect of ring-shaped defect on magnetic vortex cores(1) The final steady oscillation radius Rv and the oscillation frequency f are increased as current density increases for regular nanodisks.(2) When ring-shaped defect is a attractive potential, vortex core can through the ring, the Rv and the f have a regulation by changing inner radius r, and the adjustment range of f is about 70 MHz.(3) When ring-shaped defect is a repulsive potential, vortex core can through the ring, inner radius r, width L, exchange stiffness constant Ar and saturation magnetization Ms of ring can regulate the Rv and the f(the maximum of 640 MHz) for the case of without perpendicular magnetic anisotropy(PMA) of ring. With PMA of ring, vortex core can through the ring when L is smaller, while the polarity of vortex core reverses in the inner border of ring. The f has a bigger regulation by changing r and Ar, while it is small for L and PMA constant.3. The influnce of a vacancy on the dynamics of magnetic vortex coresThis section investigated the influnce of a vacancy on vortex cores precession in nanodisks using a GPU-accelerated micromagnetic simulation program-MuMax3. When the distance r between vacancy and geometric center varies, the precession appears different situation. Firstly, when r = 25 nm, the vortex core is pinned. Next, when the r changes in range of 35 to 75 nm, the time needed for periodic oscillation is set to T, which gradually increases with the increase of r, and the precession frequency f has a fine tuning in range of 10 MHz. It is important that when r = 35 nm, the adjustment range of the f increases to 725 MHz as the size of nanodisks increases. Finally, for r = 75 nm, the T of the vortex is about 1/2 times higher than that of without vacancy, and the needed current density is smaller.