Spin Dynamics in Various Time Regimes

Abstract

Magnetization dynamics has attracted considerable attention in many applications such as next generation storage, logic, and communication devices as well as in fundamental physics. Precession of the magnetization excited by ultrafast demagnetization emits spinwaves even in the absence of any electrical current, which can be used as an information carrier. Controlling the magnetization by current has also been opened the research field of spintronics, enabling fast magnetization switching or domain wall motion by torques induced by conduction electrons. To investigate the magnetization dynamics occurs in various time regimes, we develop the microscope system whose temporal resolution can resolve the dynamics in picoseconds scale. Another microscope system to study relatively slow time regime is also demonstrated. Two measurement systems adopt magneto-optical Kerr effect and high signal-to-noise ratio of systems enables a resolution of the magnetization dynamics to be within few degrees. From the point of view of time regimes in which the dynamics occurs, precessional motion of magnetization at the pump beam laser spot and propagating spinwaves from the pump beam spot are observed in the time regime from femtosecond to nanosecond. Distinct propagation behavior according to the direction between wave vector and magnetization is explained by different modes of excited spinwaves. Experimental results are also verified by micromagnetic simulation. In a longer time scale, current induced magnetization dynamics is studied. Spin torque, which is the torque exerted on the magnetization by electrical current, by short-current pulses generated a telegraph noise under the external magnetic field, which comes from successive depinnings of domain wall. It is shown that energy barrier is extracted and spin-torque efficiency can be determined from the probability of domain wall depinning. We also demonstrate an optical method to study magnetization dynamics induced by spin-orbit torque. The magnetization dynamics measured at quasistatic time regime is obtained by use of MOKE microscope system and, furthermore, we introduce the microscope system with circularly polarized light so that detected signals are free from the planar effect. In this method, spin-orbit torque efficiency of various samples whose structures are systematically changed is investigated. Our findings in this thesis provide understanding of the underlying physics in magnetization dynamics triggered by different mechanisms as well as provide the alternative quantification method in current induced dynamics.