Optical studies of current-induced spin-orbit effects in magnetic systems

Efficient electrical control of magnetic moments is essential for future spintronics applications, in which the intrinsic spin of the electron is utilized in addition to the electron charge for data processing and storage. Spin-orbit-interaction-induced phenomena, including the spin Hall effect, Rashba-Edelstein effect and the resultant spin-orbit torques (SOTs), have fueled the development of spintronics for more than a decade thanks to their promising magnetization-switching efficiencies. This dissertation presents experimental studies of current-induced novel phenomena arising from the spin-orbit interaction in magnetic materials. We first develop a highly-sensitive SOT magnetometer system based on the magneto-optic Kerr effect (MOKE). With the help of this sensitive system, we find an anomalous spin-orbit torque (ASOT) at the surfaces of single-layer magnetic thin films. Following the insight provided by the single-layer findings, we further demonstrate self-spin-orbit torque (SSOT) in multilayer systems lacking traditional nonmagnetic spin-source materials. A simple and accurate SOT characterization approach is important not only for scientific research, but also for industrial product development. We develop a SOT magnetometer system employing MOKE and lock-in detection for high-sensitivity spin-orbit torque measurements. By controlling the incident light polarization in the normal-incidence configuration, polar- and quadratic-MOKE are used to measure out-of-plane and in-plane SOT-induced magnetization reorientation, respectively. Thanks to its simplicity and high sensitivity ($< 70$ $\mathrm{nrad/\sqrt{Hz}}$ polarization rotation resolution), our SOT magnetometer system enables us to perform a variety of experiments for discovery of new phenomena. A well-known spin-orbit-interaction-induced phenomenon in magnetic materials is the anomalous Hall effect (AHE). In this dissertation, we report the observation of a counterpart of the AHE that we term the ASOT, wherein an electric current parallel to the magnetization generates opposite spin-orbit torques on the surfaces of the magnetic film. After a series of thickness-dependent and interface-varying experiments on different magnetic materials, we interpret the observed ASOT as due to a spin-Hall-like current generated with a high efficiency. This work leads to the conclusion that a single-layer ferromagnet can generate SOTs on its own surfaces, which introduces a new route for electrically manipulating magnetization in magnetic nanodevices. Current-induced SOTs in multilayer structures consisting of a ferromagnetic metal (FM) and a nonmagnetic spin-source material (SSM) can efficiently manipulate the magnetization and magnetic textures of the FM. The origin of the SOT is often attributed to the spin current generated by the nonmagnetic SSM, which generates a spin transfer torque on the FM. In light of our study of ASOT in single-layer magnetic films, we examine the effects of an FM-originated spin current in multilayer structures. It turns out that such spin current leads to large SOTs on the FM itself. We refer to this long-overlooked SOT as the self-spin-orbit torque (SSOT). The discovery of SSOT provides a new method for manipulating magnetization by using magnetic materials that work with nonmagnetic SSMs constructively