[论文解读] Large and Robust Charge-to-Spin Conversion in Sputtered Weyl Semimetal WTex with Structural Disorder

Xiang Li, Peng Li+11 more|arXiv (Cornell University)|Jan 13, 2020

Magnetic and transport properties of perovskites and related materials被引用 5

一句话总结

本研究在具有结构无序的溅射Weyl半金属WTe_x薄膜中实现了大且稳定的电荷-自旋转换，其自旋霍尔电导率和电荷-自旋转换效率优于单晶WTe₂。该薄膜在室温下以0.97 MA/cm²的低电流密度实现了无场磁化翻转，凸显其在工业规模自旋电子器件中的应用潜力。

ABSTRACT

Topological insulators have recently shown great promise for ultralow-power spin-orbit torque (SOT) devices thanks to their large charge-to-spin conversion efficiency originating from the spin-momentum-locked surface states. Weyl semimetals, on the other hand, may be more desirable due to their spin-polarized surface as well as bulk states, robustness against magnetic and structural disorder, and higher electrical conductivity for integration in metallic magnetic tunnel junctions. Here, we report that sputtered WTex thin films exhibit local atomic and chemical structures of Weyl semimetal WTe2 and host massless Weyl fermions in the presence of structural disorder at low temperatures. Remarkably, we find superior spin Hall conductivity and charge-to-spin conversion efficiency in these sputtered WTex films compared with crystalline WTe2 flakes. Besides, the strength of unidirectional spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is up to 20 times larger than typical SOT/ferromagnet bilayers reported at room temperature. We further demonstrate room temperature field-free magnetization switching at a current density as low as 0.97 MA/cm2. These large charge-to-spin conversion properties that are robust in the presence of structural disorder and thermal annealing pave the way for industrial production of Weyl semimetals. Our results open up a new class of sputtered Weyl semimetals for memory and computing based on magnetic tunnel junctions as well as broader planar heterostructures containing SOT/ferromagnet interfaces.

研究动机与目标

• 开发一种可扩展、适合工业应用的制备方法，用于生产具有高自旋-轨道转换效率的Weyl半金属薄膜。
• 研究结构无序与热退火对Weyl半金属中电荷-自旋转换的影响。
• 利用溅射WTe_x在室温下实现高效、无场磁化翻转，应用于磁隧道结。
• 展示无序Weyl半金属体系中自旋霍尔电导率与自旋-轨道力矩的鲁棒性。

提出的方法

• 通过溅射制备WTe_x薄膜，使其在宏观结构无序下仍保持局域原子与化学有序性，从而形成Weyl半金属相。
• 通过WTe_x/Mo/CoFeB异质结中的自旋霍尔磁阻（SMR）测量自旋霍尔电导率与电荷-自旋转换效率。
• 采用热退火处理以提升结构与电子性能，同时保持Weyl费米子态。
• 在室温下通过电流脉冲在WTe_x基磁隧道结中进行无场磁化翻转实验。
• 将溅射WTe_x薄膜的自旋霍尔电导率与SMR响应与高质量单晶WTe₂薄片的性能进行对比。
• 利用单向自旋霍尔磁阻量化异质结中的自旋-轨道力矩效率。

实验结果

研究问题

• RQ1 尽管存在结构无序，溅射WTe_x薄膜是否仍能容纳无质量Weyl费米子？
• RQ2 溅射WTe_x薄膜的电荷-自旋转换效率与单晶WTe₂薄片相比如何？
• RQ3 热退火在多大程度上提升了无序Weyl半金属薄膜中的自旋霍尔电导率与自旋-轨道力矩？
• RQ4 能否在室温下以低电流密度实现WTe_x基异质结的无场磁化翻转？
• RQ5 溅射Weyl半金属在结构无序与磁性无序下，其自旋-轨道特性具有多强的鲁棒性？

主要发现

• 溅射WTe_x薄膜表现出与Weyl半金属WTe₂一致的局域原子与化学结构，即使在结构无序下仍能容纳无质量Weyl费米子。
• 溅射WTe_x薄膜的自旋霍尔电导率与电荷-自旋转换效率优于单晶WTe₂薄片。
• 退火处理后的WTe_x/Mo/CoFeB异质结中，单向自旋霍尔磁阻在室温下比典型SOT/铁磁体双层结构高出最多20倍。
• 在室温下，WTe_x基磁隧道结中实现了低至0.97 MA/cm²电流密度的无场磁化翻转。
• 溅射Weyl半金属中的电荷-自旋转换特性对结构无序与热退火具有鲁棒性，支持可扩展器件集成。
• 溅射Weyl半金属薄膜为高性能自旋电子器件的工业化生产提供了可行平台。