电流调控磁涡旋的极性和旋性

Controlling of magnetic vortex chirality and polarity by spin-polarized current

提出了一种特殊自旋阀结构,其极化层(钉扎层)磁矩沿面内方向,自由层磁矩成磁涡旋结构.自由层在形状上设计成左右两边厚度不同的阶梯形圆盘.微磁学模拟研究发现,通过调控所施加的高斯型脉冲电流的大小、方向和脉冲宽度,可以实现磁涡旋的不同旋性、不同极性的组态控制.分析了该结构中电流调控磁涡旋旋性和极性的物理原因和微观机理.

For a nanodisk, magnetic vortex characterized by a curling magnetization is an energetically stable state. The magnetization in the center of the magnetic vortex is directed upward or downward, namely, the vortex core polarity p = ＋1 or p =-1 refers to up or down, respectively. The curling direction of magnetization, namely, the vortex chirality,is either counter-clockwise or clockwise. Thus, different combinations of chirality and polarity in a vortex structure demonstrate four stable magnetic states, which can be used to design a multibit memory cell. Such a multibit memory application requires the independent controlling of both the vortex chirality and vortex polarity, which has received considerable attention recently. Switching the vortex polarity has been achieved by using either a magnetic field or a current. The vortex chirality can be controlled by introducing asymmetric geometry of nanodisks. In this article,by using micromagnetic simulations, we present an effective method to simultaneously control the vortex chirality and polarity in a spin valve structure, in which the fixed spin polarizer layer is magnetized in the film plane when the free layer has a magnetic vortex configuration. The free layer is designed into a ladder shape with the right part being thicker than the left part. Our simulations indicate that a combination of desirable vortex chirality and polarity can be easily controlled by a Gaussian current pulse with proper strength and pulse duration through the spin-transfer torque effect.The insight into physical mechanism of the controllable vortex is demonstrated by a series of snapshots. If the magnetic moment of the free layer is saturated in the direction of 0〈 θ〈 π during the current pulse, where θ is the angle between the magnetization and ＋x axis, the vortex with the counter-clockwise chirality will be generated after the pulse. In contrast, if the free layer magnetization is saturated along the direction π 〈θ 〈2π, after the pulse, the vortex will have the clockwise chirality. The core polarity of the remanent vortex state is determined by the sign of the magnetic charges which are formed in the step-side of nanodisk during the current pulse.