Voltage-Controlled Effects in Magnetic Tunnel Junctions

Abstract

Spintronics, different from traditional electronics, explores additional routes to manipulate electrons through its spin degree of freedom. During its development, magnetic tunnel junctions (MTJs) have been playing a significant role. This is partially due to their fantastic properties such as symmetry-conserving tunneling, which leads to very large magnetoresistance (TMR) and sensitive detection of spin configuration. The relatively simple core structure of the ferromagnet/tunneling barrier/ferromagnet makes it possible to incorporate MTJs into the well-developed fabrication process and a large variety of material systems. Moreover, in recent years, the voltage-controlled effects discovered at FM/Oxide interfaces enables researchers to explore magneto-electric properties of MTJ and more functionalities have been proposed and verified especially in magnetic switching. In this work, we focus our attention on the voltage-controlled effects based on the conventional CoFeB/MgO magnetic tunnel junctions. After a brief introduction to the fundamentals of spin-dependent transport and perpendicular magnetic anisotropy (PMA) in the CoFeB/MgO MTJs. We divert our efforts to various voltage-controlled mechanism discovered in recent years. Chapter 2 briefly touches on the basic experimental technique used in this work. In Chapter 3, we will introduce a revised MTJ nanopillar fabrication procedure which dramatically lowers the requirement for fabricating high-quality MTJ and increases the yield significantly. Chapter 4 will involve interfacial engineering at CoFeB/MgO interface in MTJs. By dusting a thin layer of heavy metal (Ir), in addition to controlling the annealing condition, the VCMA coefficient can be significantly enhanced and the voltage induced switching energy can be greatly reduced, which demonstrated the promises to incorporate MTJs into next-generation memory applications. In Chapter 5, a novel voltage-controlled exchange bias (VCEB) effect is reported and its implication on magnetic switching and further enhancement of TMR is discussed. The last chapter will briefly cover the potential improvement of finished work and topics for future research.