垂直自由层倾斜极化层自旋阀结构中的磁矩翻转和进动

在理论上研究了垂直自由层和倾斜极化层自旋阀结构中自旋转移矩驱动的磁矩翻转和进动.通过线性展开包括自旋转移矩项的Landau-Lifshitz-Gilbert方程并使用稳定性分析方法,得到了包括准平行稳定态、准反平行稳定态、伸出膜面进动态以及双稳态的磁性状态相图.发现通过调节电流密度和外磁场的大小可以实现磁矩从稳定态到进动态之间的转化以及在两个稳定态之间的翻转.翻转电流随外磁场的增加而增加,并且受自旋极化方向的影响.当自旋极化方向和自由层易磁化轴方向平行时,翻转电流最小;当自旋极化方向和自由层易磁化轴方向垂直时,翻转电流最大.通过数值求解微分方程,给出了不同磁性状态磁矩随时间的演化轨迹并验证了相图的正确性.

Magnetization reversal and precession in spin valve structures with a perpendicular free layer and a tilted polarizer layer

Spin-transfer effects induced by spin-polarized current in the spin valve structures present a platform for studying different static and dynamic magnetization states sustained or driven by current. Especially, it can excite some new magnetic states and cause magnetization reversal and precession, which offers some promising applications in data processing and microwave emission. However, most of researches so far have focused on the spin valve structure with parallel or perpendicular anisotropy. Compared with the spin valve structure with parallel or perpendicular anisotropy device, the spin valve structure with a tilted polarizer is also hopeful for its potential application in fast-switching and high-density magnetic recording. Moreover, the tilted polarizer provides a new way to control the spin torquedriven magnetization dynamics in spin valve structure. In this paper, the magnetization reversal and precession driven by the spin-transfer torque in spin valve structures with a perpendicular free layer and a tilted polarizer layer are investigated theoretically. By linearizing the Landau-Lifshitz-Gilbert equation including the spin-transfer torque, two coupled dynamically evolutive equations and new equilibrium directions are obtained. Performing stability analysis for all new equilibrium directions and taking Co/Ni]×4 multilayers as an illustrative example, we obtain the phase diagrams of magnetic states defined in parameter space spanned by external magnetic field and current density. Several magnetic states, including quasi-parallel stable states, quasi-antiparallel stable states, out-of-plane precession, and bistable states are distinguished in the phase diagrams. Through adjusting the magnitudes of current density and external magnetic field, the switching from stable states to precessional ones and the reversal between two stable states can be realized, and the reversal current increases with the external magnetic field increasing. Meanwhile, we portray the phase diagram of magnetic states defined in parameter space spanned by current density and the direction of tilted polarizer. In this case, the out-of-plane precession does not emerge as the current density and external magnetic field are relatively small. Affected by the directions of spin polarizer, the reversal current of magnetization is lowest when the direction of spin polarizer is parallel to the easy axis of free-layer, and is largest when the direction of spin polarizer is perpendicular to the easy axis of free-layer. Selecting the different directions of the polarized-layer magnetization provides an alternative way to improve the efficiency of current-driven microwave emitting and magnetization reversal. By solving temporal evolution equations numerically, the behaviors of different magnetic states are shown and the validities of the phase diagrams are confirmed.