A study of spin-orbit torques and deterministic field-free switching of perpendicular magnetization

Spintronic devices provide an energy-efficient platform for implementing non-volatile memory and logic. In spintronic memory devices, the information is stored in the magnetization state of the free magnetic layer, and current, voltage or strain-induced mechanisms are used for switching the magnetization. Among the operation methods, current-induced spin-orbit torque (SOT) is a promising mechanism for magnetization switching with faster dynamics, higher endurance, and potentially higher energy efficiency compared to the conventional spin-transfer torque. For magnetic memory applications, perpendicular magnetic anisotropy is desirable since it enables high memory densities. However, for deterministic SOT switching of a perpendicular magnet, an external magnetic field collinear with the current is required, which hampers the applicability of SOT switching. Although there have been several prior effects aimed at solving this problem, but most involve structural asymmetries or additional layers that are not practical for large wafer scale applications. Consequently, a practical realization of deterministic field-free SOT switching of perpendicular magnetization remains a challenge.In this dissertation, we present two main methods for realizing deterministic field-free SOT switching for practical uses. First, we show that the external in-plane field can be replaced by a built-in exchange bias field using antiferromagnetic materials. We also show that certain antiferromagnets can create the SOTs themselves, serving as the layer providing both the exchange bias and SOTs. As the second approach, we use the concept of structural asymmetry reported previously, and modify the conventional SOT heterostructure by inserting a slightly asymmetric light-metal at the heavy-metal/ferromagnet interface. The broken structural symmetry enables the creation of current-induced out-of-plane effective magnetic fields, which break the symmetry between the up and down states for each current polarity and allow for deterministic SOT switching at zero external magnetic field. We also apply the asymmetry concept to a second material system with a minimal structural asymmetry, resulting in an enhanced magnetic uniformity across large wafer areas. We show that the latter approach for field-free SOT switching has almost all the characteristics of a practical solution that could be used in applications. We also present a better understanding of the deterministic switching process enabled by the structural asymmetry by examining its microscopic origins. In the last part of this dissertation, we discuss several other interesting aspects of the antiferromagnet-based material system (IrMn/CoFeB/MgO) that we primarily developed for the field-free SOT switching. We show that this material system has several unique properties, including the simultaneous presence of Dzyaloshinskii-Moriya interaction, voltage-control of magnetic anisotropy (VCMA), exchange bias field, and spin-orbit torques. We use this material system as a platform for studying magnetic skyrmions, where we can create/annihilate skyrmions using single voltage pulses without any external magnetic fields via the VCMA effect. Furthermore, we use spin-orbit torques to move the skyrmions in the same structure, providing a promising platform for skyrmion-based device applications