Nonequilibrium Phenomena in Novel Magnetic Materials

所提出的研究集中在磁化动力学的各个方面，磁化动力学发生在磁系统被驱离其平衡态后的几纳秒内。研究成果解决了从基本问题“磁性元素中的磁化能以多快的速度转换？”与应用相关的问题：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。由于此类材料在磁随机存取存储器和高密度磁存储等自旋电子学应用中的重要性，因此得到了广泛的研究。我们将研究垂直磁化的锰基Heusler合金的磁性，其中进动动力学将用亚皮秒时间分辨率的时间分辨磁光来探测。该研究还包括对MnAl薄膜自旋重取向转变的动态过程的研究。 由自旋极化电流激发的涡旋动力学也令人感兴趣。以前，我们已经报道了NiFe/Cu/Co圆形纳米管中的自旋转移扭矩效应，其中涡旋手性可以通过注入电流脉冲来控制切换。最近，我们通过国际合作成功地制造了这样的纳米磁体。涡旋动力学的细节，例如自旋波本征模的激发，将用高频磁阻测量来研究。从应用的角度来看，这一研究成果可能为涡旋纳米管在存储器件中的应用提供机会，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，以阐明自旋转移扭矩诱导的磁化动力学，并准确地预测上述纳米元素的磁化过程，以改进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.