Nonequilibrium Phenomena in Novel Magnetic Materials

拟议的研究集中在磁化动力学的各个方面，这发生在磁系统被赶出其平衡状态后的几纳秒内。研究成果解决了从“磁性元件的磁化转换速度有多快？”如何操纵自旋以应用于高性能/高密度/低能耗的磁电子器件？研究目标之一是探索具有大垂直磁各向异性的新型铁磁材料。由于这些材料在自旋电子学应用中的重要性，如磁性随机存取存储器和高密度磁性存储器，它们正在被广泛研究。我们将研究垂直磁化的Mn基Heusler合金的磁性，其中的进动动力学将使用时间分辨的时间分辨率为亚皮秒的磁光探测。本论文还对MnAl薄膜中自旋重取向转变的动力学过程进行了研究，并对自旋极化电流激发的涡旋动力学进行了研究。在此之前，我们已经报道了NiFe/Cu/Co圆形纳米柱中的自旋转移矩效应，其中涡旋手性可以通过注入电流脉冲来可控地切换。最近，我们通过国际合作成功地制造了这种纳米磁体。涡流动力学的细节，如自旋波本征模的激发，将使用高频磁阻测量进行研究。从应用的角度来看，该研究成果可能会带来在存储器件应用中实现涡旋纳米柱的机会，其中具有可控手性切换的涡旋态可以用作存储位。作为实验的补充，将进行详细的数值模拟，以阐明自旋，转移转矩诱导的磁化动力学，并且还精确地预测上述纳米元件中的磁化过程，以改善所提出的自旋电子器件的性能。

The proposed research is focused on various aspects of magnetization dynamics, which occurs within a few nanoseconds after the magnetic systems are driven out of their equilibrium states. The research outcomes address the issues ranging from the fundamental question "How fast can the magnetization in magnetic elements be switched?" to the application-related problem "How to manipulate spins for the application of high-performance/high-density/low energy-consuming magentoelectronic devices?".One of the research objectives is to explore novel ferromagnetic materials for large perpendicular magnetic anisotropy. Such materials are being extensively studied due to their importance in spintronics applications like magnetic random access memory and high-density magnetic storage. We will investigate the magnetic properties of perpendicularly magnetized Mn-based Heusler alloys, in which the precessional dynamics will be probed using time-resolved magneto-optics with temporal resolution of sub-picosecond. The proposed research also includes the study of the dynamic process of the spin reorientation transition in MnAl thin films.The vortex dynamics excited by spin-polarized current is also of interest. Previously, we have reported on the spin-transfer torque effect in NiFe/Cu/Co circular nanopillars, in which the vortex chirality can be controllably switched by injecting current pulses. Recently, we have successfully fabricated such nanomagnets through an international collaboration. The details of the vortex dynamics, such as excitation of spin-wave eigenmodes, will be investigated using high-frequency magnetoresistance measurement. From an application point of view, the research outcome may lead to an opportunity to implement the vortex nanopillars in memory device applications, in which vortex states with controllable chirality switching may be used as memory bits.Complementary to the experiments, detailed numerical modeling will be performed in order to elucidate the spin-transfer torque induced magnetization dynamics and also to accurately predict the magnetization process in above described nanoelements to improve the performance of proposed spintronics devices.