Spin-Transfer Torque and Spin-Orbit Torque Driven Dynamics of Chiral Domain-Walls

Abstract

Controlling domain-walls (DWs) by electric current promises the realization of future spintronic memory and logic devices. Since the first theoretical possibility of manipulating magnetic DWs by electric currents was suggested in the 1980s, now it is found that there are three important ingredients for the current-driven domain wall-motion, that is, spin-transfer torque (STT), spin-orbit torque (SOT) and Dzyaloshinskii-Moriya interaction (DMI). In this thesis, the current-driven DW motion in perpendicularly magnetized Pt/Co/Pt system was investigated in the framework of these up-to dated mechanisms. For this study, scanning magneto-optical Kerr effect (MOKE) microscope, equipped with out-of plane and in-plane electromagnets, was developed to observe the magnetic domain and DW dynamics. The out-of-plane electromagnet was used to drive the DW motion and the in-plane electromagnet was used to control the DW structure which is crucial factor for the effect of SOT on DW. We first investigate the DW structure dependence of magnetic field-driven DW motion. Interestingly, we observe a symmetry-breaking in DW dynamics under in-plane bias field. This symmetry-breaking is found to be caused by the existence of DMI in Pt/Co/Pt system. From the observation of asymmetric DW dynamics, we developed a simple method to evaluate the built-in DW chirality stabilized by DMI. Based on the determined DW chirality, we then succeed in separating both effects of STT and SOT. From the analysis, we found that there exists a new degree of freedom in STT—the negative STT (nSTT)—that pushes DWs to the opposite direction of standard STT mechanism. The efficiency of nSTT is found to be comparable to that of SOT, signalling the possibility of a promising operation mechanism for the emerging spintronic devices. As an effort in the application, we also study the extrinsic method to obtain single domain pattern which is important for the simple operation of DW motion-based devices. By reducing the width of ferromagnetic wire, we observe the transition from dendrite to single domain that gives the breakthrough in the limit of intrinsic material properties. Our findings in this thesis provide the latest understanding of current-driven DW motion in ultrathin Pt/Co/Pt system and trigger further researches about the asymmetric field-driven DW motion and engineering DMI and STT in various materials.