FeCr合金薄膜中的超快磁动力学研究

The study of magnetization dynamics has attracted a lot of attentions in the last two decades owing to the growing demands to increase the speed of controlling the magnetic state in magnetic recording media and spintronic devices. Recent experimental studies have revealed a new approach to manipulate the magnetization of magnetic materials on subpicosecond time scales by femtosecond laser pulses. More, a great deal of research has been dedicated to all-optical magnetization reversed materials in the past few years since its magnetization switching has been experimentally observed in subpicosecond time scale by femtosecond laser pulses. One of the prior work for understanding all-optical magnetization reversal and designing faster magnetic data-storage and spintronic devices based all-optical magnetization reversal is to seek new magnetic materials which can be all-optical magnetization reversed via exploring the correlation between the antiferromagnetic interaction of two sublattices and the disspation of spin angular momentum. This project will use an all optical pump-probe MOKE to probe the dynamics of the ultrafast demagnetization in FeCr alloy films. The antiferromagnetic interaction between Fe and Cr atoms can be tuned by varied content of Cr in ferrimagnetic FeCr films. Therefore we can explore the correlation between the ultrafast demagnetization and the antiferromagnetic interaction which is the key issue to understanding the spin angular momentum transfer between two sublattices.This work will help us to seek new magnetic materials which can be all-optical magnetization reversed among FeCr alloys.

由于磁存储和自旋电子学器件对速度的要求越来越高，磁动力学的研究在最近几十年是磁性材料研究的重点。最近发现采用飞秒激光脉冲可以使得磁性金属材料的磁矩在亚皮秒尺度退磁，提供了一种新的操纵自旋手段。尤其是在亚铁磁材料中发现超快的全光磁反转，导致飞秒磁学成为研究热点。通过研究反铁磁耦合对自旋角动量耗散的影响去寻找新的全光磁反转材料并，是研究全光磁反转的动力学机制以及未来全光磁反转器件发展的基础。本研究项目拟采用飞秒激光全光学泵浦探测的方法来研究亚铁磁FeCr合金薄膜中的超快退磁行为，重点是通过调节Cr的含量后，研究Fe-Cr间的反铁磁耦合对超快退磁的影响，来理解自旋角动量在亚铁磁次格子间的转移，并在FeCr合金薄膜中寻找到新的全光磁反转材料。

由于可以在亚皮秒尺度实现磁反转，最近亚铁磁全光磁反转引起了极大关注。对于全光磁反转的研究而言，核心是探究两个子系统间由于反铁磁耦合和自旋角动量耗散的关联。在此项目中，我们利用时间分辨的TRMOKE和铁磁共振，研究了FeCr亚铁磁薄膜中的磁动力学。在FeCr合金薄膜中，提高Cr的原子占比，我们发现其临界温度和自旋波交换劲度系数都降低，这也符合平均场理论。其超快退磁时间也随着Cr含量的增加而减小，甚至出现了一个慢的，类似两步退磁的信号。前者是由于Fe-Cr原子的交换耦合，增加了Fe的自旋角动量耗散。自旋从Fe转移到Cr，再在Cr上面弛豫，这样原子间的超快自旋流加速了超快退磁过程。两步退磁和FeCr体系中声子体系慢的热弛豫有关。此外，我们还在顺磁态成分的FeCr合金薄膜中，观察到了激光诱导的瞬态铁磁信号。除了原子间的反铁磁耦合体系外，我们还研究了存在反铁磁耦合或是磁矩不平行的其它体系，如铁磁/反铁磁异质结Fe/RFeO3, FeGa/IrMn体系中，铁磁和反铁磁的交换作用导致了超快自旋流，并引发了太赫兹自旋进动和退磁变快等行为；在Co/Pt多层膜中，发现多畴态对超快退磁不大；在自旋阀结构中，钉扎层磁化从反平行切换到平行状态对的过程中，自旋流不变。在FeGe/Fe体系中，非共线磁结构和层间耦合导致了大的自旋流。我们的结果表明自旋转移所引发的自旋流对超快退磁和全光磁反转的理解十分重要，同时启迪我们可以利用反铁磁耦合或是非平行排列来有效在自旋电子学器件中调控自旋流。