Study Of Dynamic Characteristics Of Bias-field-free Spin Torque Nano-oscillator

Spin transfer torque(STT) induced magnetization switching and oscillation in nanometer scale magnetoresistance(MR) devices has been studied intensively due to its direct application in the non-volatile STT random access memory(STT-RAM) and its potential application in the high frequency spin torque oscillatior(STO). STO could be used in highdensith microwave signal processor and chip-to-chip communication system due to its nanometer scale footprint and ultra high oscillation frequency. STO has also been suggested in the magnetic recording head for the microwave assisted magnetic recording(MARM) and in the high-speed magnetic reader for furture hard disk drive. However, several critical engineering challenges for these exciting STO applications are still remaining, including improving the output power, and removing the applied magnetic field for STO operation. In order to improving the output power, recently a spin transfer oscillator with a perpendicularly magnetized free layer and in-plane magnetized pinned layer has been fabricated and obtained the large output power. However, there is a large remainin issue that the application of a bias magnetic field is necessary.In this thesis work, the magnetization dynamics of spin torque oscillator(STO)consisting of a perpendicularly magnetized free layer and in-plane magnetized pinned layer was studied. And discuss the effect of second-order perpendicular anisotropy, field-like spintransfer torque, and in-plane shape anisotropy on the oscillation characteristics. The main research contents and results are given below:1. We derived the analytical formula for the threshold current, current dependence of the oscillation frequency of the STO. The derivation is based on the analysis of the energy balance between the work by the spin torque and the energy dissipation due to the damping.We find that an external magnetic field is requited to obtain precession. And the oscillation frequency monotonically decreases with increasing the current. The validity of the analytical solution was confirmed by numerical simulation based on the Landua-Lifshitz-GilbertSlonczewski(LLGS) equation, the analytical results are well reproduced by the numerical simulation.2. We take a approach to obtaining bias-field-free oscillation in the STO with the perpendicularly magnetized free layer with first- and second-order uniaxial anisotropy. Using the macrospin analysis, we show that the second-order uniaxial anisotropy can steady-atate oscillation even in the absence of a bias field. We derive analytical expressions for the threshold current, and current dependence of the oscillation frequency based on the analysis of the energy balance between the work by the spin torque and the energy dissipation due to the damping. The analytical results are confirmed to the agree with numerical simulations.3. We then take another approach to obtaining bias-field-free oscillation in the oscillation by taking into account the field-like spin-transfer torque. It is shown that the field-like torque plays an important role in finding the energy balance between the energy supplied by the spin torque and the dissipation due to the damping, which results in a steady precession in the zero bias magnetic field. The oscillation characteristics for various field-like torque were also studied by numerical simulation.4. The influence of the intrinsic in-plane shape anisotropy on the magnetization dynamics of the STO is studied numerically based on the LLGS equation. It is shown that in the presence of in-plane shape anisotropy, self-oscillation is induced even at zero bias magnetic field. We derive analytical expressions for the threshold current, and current dependence of the oscillation frequency based on the analysis of the energy balance. The oscillation characteristics for various in-plane shape anisotropies were also studied by numerical simulation. We find that a relatively large current region of zero-field selfoscillation, in which the corresponding microwave frequency is increased but the threshold current still maintains an almost constant value, can be obtained by introducing a relatively large intrinsic in-plane shape anisotropy.