Department of Materials Science EngineeringElectrical manipulation of magnetization has grown as an essential ingredient in rapidly evolving spintronic research. Switching of nano-scale magnetization can be induced by a spin-polarized current via spin-transfer torque, domain wall motion, and/or spin-orbit torque, which are being increasingly utilized for magnetic memory devices under development. Apart from current dissipation, the electric field itself can also be used to control the magnetism in various materials, especially in dilute magnetic semiconductors (DMSs). A gate-voltage-induced accumulation of charge could alter magnetic exchange interactions and eventually lead to changes in magnetic moment, coercivity, anisotropy, and transition temperature. Semiconductor spintronics has garnered increasing attention due to the concept behind the spin field-effect transistor (spin-FET), where the spin precession is governed by the gate-controllable Rashba field. Tuning the magnetization of the source and drain in the spin-FET architecture offers additional state variables in future state-of-the-art electronic applications.
This dissertation addresses the study of dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). The ZnO NWs used in this study were grown by using chemical vapor deposition (CVD) technique. The crystal structure and composition of ZnO NWs were studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). Ferromagnetism in our ZnO NWs arose from oxygen vacancies, which constitute deep levels hosting unpaired electron spins. The magnetic transition temperature of the studied ZnO NWs was estimated to be well above room temperature. The in situ UV confocal photoluminescence (PL) study confirmed oxygen vacancy mediated ferromagnetism in the studied ZnO NW FET devices. Both the estimated carrier concentration and temperature dependent conductivity reveal the studied ZnO NWs are at the crossover of the metal-insulator transition. In particular, gate-induced modulation of the carrier concentration in the ZnO NW FET significantly alters carrier-mediated exchange interactions, which causes even inversion of magnetoresistance (MR) from negative to positive values. Upon sweeping the gate bias from −40 V to +50 V, the MRs estimated at 2 T and 2 K were changed from −11.3% to +4.1%. Detailed analysis on the gate dependent MR behavior clearly showed enhanced spin splitting energy with increasing carrier concentration. Gate voltage dependent PL spectra of an individual NW device confirmed the localization of oxygen vacancy-induced spins, indicating that gate-tunable indirect exchange coupling between localized magnetic moments played an important role in the remarkable change of the MR.ope
