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arXiv 1702.04155cond-mat.mes-hall
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基于压缩的自旋霍尔纳米振荡器中磁化自振荡的起源

On the origin of magnetization auto-oscillations in constriction-based spin Hall nano-oscillators

Mykola Dvornik, Ahmad A. Awad, Johan Åkerman

摘要:

我们使用微磁模拟映射出和比较,线性和自动振荡模式的收缩为基础的自旋霍尔纳米振荡器作为施加的磁场的函数,具有不同的幅度和面外角度。我们证明,对于所有可能的应用领域的配置的自振荡出现从本地化的线性模式的收缩。对于趋向于平面的场方向,这些模式是所谓的“边缘”类型,即,局限于收缩的相对侧。当磁化方向相反地接近膜法线时,模式转变为所谓的“体”型,即,定位在具有显著增加的进动体积的收缩部内部,这与磁荷从收缩部的侧面到顶表面和底表面的重新分布一致。在一般情况下,相应的自振荡的阈值电流增加与所施加的电场强度和减少与它的出平面角,符合的行为的内部字段和一个macrospin模型。然后通过考虑到系统的强非均匀性,通过平均场近似实现定量协议。Oe场和来自驱动电流的自旋转移力矩都增加了所观察到的模式的局部化并降低了频率。此外,反对称Oe场打破了横向对称性,有利于在两个收缩边缘之一的局部模式,特别是对于大的面外场角,其中阈值电流显着增加,边缘退磁被抑制。

英文摘要:

We use micromagnetic simulations to map out and compare, the linear and auto-oscillating modes in constriction-based spin Hall nano-oscillators as a function of applied magnetic field with varying magnitude and out-of-plane angle. We demonstrate that for all possible applied field configurations the auto-oscillations emerge from the localized linear modes of the constriction. For field directions tending towards the plane, these modes are of the so-called "edge" type, i.e. localized at the opposite sides of the constriction. When the magnetization direction instead approaches the film normal, the modes transform to the so-called "bulk" type, i.e. localized inside the constriction with substantially increased precession volume, consistent with the re-distribution of the magnetic charges from the sides to the top and bottom surfaces of the constriction. In general, the threshold current of the corresponding auto-oscillations increases with the applied field strength and decreases with its out-of-plane angle, consistent with the behavior of the internal field and in good agreement with a macrospin model. A quantitative agreement is then achieved by taking into account the strongly non-uniform character of the system via a mean-field approximation. Both the Oe field and the spin transfer torque from the drive current increase the localization and decrease the frequency of the observed mode. Furthermore, the anti-symmetric Oe field breaks the lateral symmetry, favoring the localized mode at one of the two constriction edges, in particular for large out-of-plane field angles where the threshold current is significantly increased and the edge demagnetization is suppressed.

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