Magnetic impurities as the origin of the variability in spin relaxation rates in Cu-based spin transport devices
Magnetic impurities as the origin of the variability in spin relaxation rates in Cu-based spin transport devices
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DOI：
10.1103/physrevmaterials.3.124409
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发表时间：
2019-12
影响因子：
3.4
通讯作者：
J. Watts;L. O’Brien;J. Jeong;K. Mkhoyan;P. Crowell;C. Leighton
中科院分区：
文献类型：
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作者：
J. Watts;L. O’Brien;J. Jeong;K. Mkhoyan;P. Crowell;C. Leighton
Elliott-Yafet自旋弛豫机制假定低自旋轨道耦合的金属中自旋和动量寿命之间的线性比例，并且在自旋电子学中被广泛接受。然而，精确的实验确定单个散射源的自旋和动量弛豫时间之间的艾略特-雅菲比例常数（βi）是具有挑战性的。这在铜的非定域自旋输运的文献中是显而易见的，例如，其中报道的声子（βph）和缺陷（βdef）艾略特-雅菲常数变化了一个数量级。在最近的工作中，我们发现，即使是百万分之一级的磁性杂质浓度可以大大影响自旋弛豫铜，通过自旋输运模拟的近藤效应。为了澄清这是否可以解释所报道的βi的变化，在这里，我们报告了对Cu基横向非局域自旋阀中自旋输运的全面研究，改变铁磁接触材料，界面结构，Cu厚度和制造后退火条件，导致广泛变化的微观结构和磁性杂质浓度。通过量化磁性杂质对电荷和自旋输运的影响，我们证明了这些杂质对自旋弛豫速率的显著甚至主导性影响，从而提取了βi。通过Al夹层插入或适度退火实现了磁性杂质效应的最小化，恢复了声子介导的自旋弛豫的预期温度依赖性，并能够更可靠地确定声子（740 ± 200）和声子缺陷（240 ± 50）的βi。后者的贡献被证明是占主导地位的晶界在这些多晶铜膜。在实际的非局部自旋阀器件的晶粒尺寸的横截面透射电子显微镜测量，然后建立一个有用的平均晶粒尺寸和自旋扩散长度之间的经验关系。这些测量突出了磁性杂质在金属自旋输运中的重要性，解释了Cu中报道的βph和βdef的广泛变化，并阐明了金属自旋输运与微观结构之间的关系。
The Elliott-Yafet spin relaxation mechanism posits linear proportionality between spin and momentum lifetimes in low spin-orbit coupling nonmagnetic metals, and is widely accepted in spintronics. Accurate experimental determination of the Elliott-Yafet proportionality constants (βi) between the spin and momentum relaxation times for individual scattering sources is challenging, however. This is apparent from the literature on nonlocal spin transport in Cu, for example, where reported phonon (βph) and defect (βdef ) Elliott-Yafet constants vary by an order of magnitude. In recent work we discovered that even part-per-million-level magnetic impurity concentrations can substantially influence spin relaxation in Cu, via a spin transport analog of the Kondo effect. To clarify whether this could explain the reported variability in βi, here we report on a comprehensive study of spin transport in Cu-based lateral nonlocal spin valves, varying the ferromagnetic contact material, interface structure, Cu thickness, and post-fabrication annealing conditions, resulting in widely varied microstructures and magnetic impurity concentrations. Quantifying the effects of magnetic impurities on charge and spin transport we demonstrate the dramatic, even dominant, effect these can have on spin relaxation rates, and thus extracted βi. Minimization of magnetic impurity effects is achieved via Al interlayer insertion or moderate annealing, restoring the expected temperature dependence for phonon-mediated spin relaxation, and enabling more reliable determination of βi for phonons (740 ± 200), and nonmagnetic defects (240 ± 50). The latter contribution is shown to be dominated by grain boundaries in these polycrystalline Cu films. Cross-sectional transmission electron microscopy measurement of grain sizes in actual nonlocal spin valve devices then establishes a useful empirical relationship between average grain size and spin diffusion length. These measurements highlight the importance of magnetic impurities in metallic spin transport, explain the wide variability in reported βph and βdef in Cu, and elucidate the relationship between metallic spin transport and microstructure.