Field-free deterministic ultrafast creation of magnetic skyrmions by spin-orbit torques
Field-free deterministic ultrafast creation of magnetic skyrmions by spin-orbit torques
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DOI：
10.1038/nnano.2017.178
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发表时间：
2017-11-01
影响因子：
38.3
通讯作者：
Beach, Geoffrey S. D.
中科院分区：
文献类型：
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作者：
Buettner, Felix;Lemesh, Ivan;Beach, Geoffrey S. D.
外磁场、杂散场能量、高阶交换相互作用和Dzyaloshinskii-Moriya相互作用（DMI）共同作用可以稳定磁天空粒子（1-6）。最后一种倾向于同手性天幕，它的运动是由自旋轨道扭矩驱动的，并且是确定性的，这使得具有大DMI的系统与应用相关。具有强自旋轨道相互作用的非磁性重金属的非对称多层和过渡金属铁磁层提供了一个大的可调谐DMI4-8。非磁性重金属层也可以通过自旋霍尔效应向铁磁层注入具有横向自旋极化的垂直自旋电流(9)。这导致扭矩（10）可用于在面外磁化的铁磁元件中完全切换磁化，但切换仅在存在对称破缺的面内场时是确定的（11-13）。尽管自旋-轨道扭矩导致连续薄膜中的畴成核（14）和磁道中skyrmions的随机成核（15），但目前还没有报道过在集成器件设计中在选定位置可控地创建单个skyrmions的实用方法。在这里，我们证明了亚纳秒的自旋-轨道扭矩脉冲可以使用与移位操作相同的电流路径在磁赛道的自定义位置确定性地产生单个skyrmions。DMI的效应表明，实现这一目标不需要外部的面内磁场。这种实现利用了一个缺陷，比如磁道中的收缩，它可以用作skyrmion发生器。这个概念适用于任何轨道几何形状，包括三维设计（16）。
Magnetic skyrmions are stabilized by a combination of external magnetic fields, stray field energies, higher-order exchange interactions and the Dzyaloshinskii-Moriya interaction (DMI)(1-6). The last favours homochiral skyrmions, whose motion is driven by spin-orbit torques and is deterministic, which makes systems with a large DMI relevant for applications. Asymmetric multilayers of non-magnetic heavy metals with strong spin-orbit interactions and transition-metal ferromagnetic layers provide a large and tunable DMI4-8. Also, the nonmagnetic heavy metal layer can inject a vertical spin current with transverse spin polarization into the ferromagnetic layer via the spin Hall effect(9). This leads to torques(10) that can be used to switch the magnetization completely in out-of-plane magnetized ferromagnetic elements, but the switching is deterministic only in the presence of a symmetry-breaking in-plane field(11-13). Although spin-orbit torques led to domain nucleation in continuous films(14) and to stochastic nucleation of skyrmions in magnetic tracks(15), no practical means to create individual skyrmions controllably in an integrated device design at a selected position has been reported yet. Here we demonstrate that sub-nanosecond spin-orbit torque pulses can generate single skyrmions at custom-defined positions in a magnetic racetrack deterministically using the same current path as used for the shifting operation. The effect of the DMI implies that no external in-plane magnetic fields are needed for this aim. This implementation exploits a defect, such as a constriction in the magnetic track, that can serve as a skyrmion generator. The concept is applicable to any track geometry, including three-dimensional designs(16).