Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures
Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures
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DOI：
10.1103/physrevb.86.024404
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发表时间：
2012-07-05
影响因子：
3.7
通讯作者：
Oppeneer, P. M.
中科院分区：
文献类型：
--
作者：
Battiato, M.;Carva, K.;Oppeneer, P. M.
飞秒激光激发铁磁材料产生具有异常输运特性的高能自旋极化电子。我们发展了一种半经典理论，专门用于捕获激光激发非平衡电子（NEQ）的传输。对随机发生的多电子碰撞引起的电子热化进行了精确的处理，并包括由于非弹性电子-电子散射而产生的电子级联。此外，发展的理论可以处理激光辐照材料中存在的几个不同层。对导出的自旋相关输运方程进行了数值求解，结果表明，热NEQ电子自旋输运既不发生在扩散态，也不发生在弹道态，而是超扩散态。在典型的过渡金属铁磁体（如Fe， Ni）中，由于受激发的自旋多数电子和少数电子具有不同的能量依赖寿命，因此产生飞秒（fs）状态下的快速自旋动力学，从而有效地产生自旋电流。作为例子，我们对典型异质结构的自旋动力学进行了数值求解，特别是铁磁/非磁性金属层状结（即Fe/Al和Ni/Al）和铁磁/非磁性绝缘体结（大带隙绝缘体上的Fe或Ni层，例如MgO）。对于激光激发铁磁层的铁磁/非磁性金属结，计算的自旋动力学表明，在非磁性层（Al）中注入了超扩散自旋电流。注入的自旋电流由经过筛选的NEQ、可移动的多数自旋电子组成，Ni的自旋极化率接近90%，Fe的自旋极化率约为65%。同时，在飞秒范围内驱动了铁磁极化的快速退磁。将产生的自旋电流类比为超扩散自旋塞贝克效应。
Femtosecond laser excitation of a ferromagnetic material creates energetic spin-polarized electrons that have anomalous transport characteristics. We develop a semiclassical theory that is specifically dedicated to capture the transport of laser-excited nonequilibrium (NEQ) electrons. The randomly occurring multiple electronic collisions, which give rise to electron thermalization, are treated exactly and we include the generation of electron cascades due to inelastic electron-electron scatterings. The developed theory can, moreover, treat the presence of several different layers in the laser-irradiated material. The derived spin-dependent transport equation is solved numerically and it is shown that the hot NEQ electron spin transport occurs neither in the diffusive nor ballistic regime, it is superdiffusive. As the excited spin majority and minority electrons in typical transition-metal ferromagnets (e.g., Fe, Ni) have distinct, energy-dependent lifetimes, fast spin dynamics in the femtosecond (fs) regime is generated, causing effectively a spin current. As examples, we solve the resulting spin dynamics numerically for typical heterostructures, specifically, a ferromagnetic/nonmagnetic metallic layered junction (i.e., Fe/Al and Ni/Al) and a ferromagnetic/nonmagnetic insulator junction (Fe or Ni layer on a large band-gap insulator as, e.g., MgO). For the ferromagnetic/nonmagnetic metallic junction where the ferromagnetic layer is laser-excited, the computed spin dynamics shows that injection of a superdiffusive spin current in the nonmagnetic layer (Al) is achieved. The injected spin current consists of screened NEQ, mobile majority-spin electrons and is nearly 90% spin-polarized for Ni and about 65% for Fe. Concomitantly, a fast demagnetization of the ferromagnetic polarization in the femtosecond regime is driven. The analogy of the generated spin current to a superdiffusive spin Seebeck effect is surveyed.