Gate-Tunable Magnetotransport in Ferromagnetic ZnO Nanowire FET Devices

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

Gate-dependent spin Hall induced nonlocal resistance and the symmetry of spin-orbit scattering in Au-clustered graphene

Engineering the electron dispersion of graphene to be spin-dependent is crucial for the realization of spin-based logic devices. Enhancing spin-orbit coupling in graphene can induce spin Hall effect, which can be adapted to generate or detect a spin current without a ferromagnet. Recently, both chemically and physically decorated graphenes have shown to exhibit large nonlocal resistance via the spin Hall and its inverse effects. However, these nonlocal transport results have raised critical debates due to the absence of field dependent Hanle curve in subsequent studies. Here, we introduce Au clusters on graphene to enhance spin-orbit coupling and employ a nonlocal geometry to study the spin Hall induced nonlocal resistance. Our results show that the nonlocal resistance highly depends on the applied gate voltage due to various current channels. However, the spin Hall induced nonlocal resistance becomes dominant at a particular carrier concentration, which is further confirmed through Hanle curves. The obtained spin Hall angle is as high as similar to 0.09 at 2 K. Temperature dependence of spin relaxation time is governed by the symmetry of spin-orbit coupling, which also depends on the gate voltage: asymmetric near the charge neutral point and symmetric at high carrier concentration. These results inspire an effective method for generating spin currents in graphene and provide important insights for the spin Hall effect as well as the symmetry of spin scattering in physically decorated graphene

Enhanced Crystallinity of Epitaxial Graphene Grown on Hexagonal SiC Surface with Molybdenum Plate Capping

The crystallinity of epitaxial graphene (EG) grown on a Hexagonal-SiC substrate is found to be enhanced greatly by capping the substrate with a molybdenum plate (Mo-plate) during vacuum annealing. The crystallinity enhancement of EG layer grown with Mo-plate capping is confirmed by the significant change of measured Raman spectra, compared to the spectra for no capping. Mo-plate capping is considered to induce heat accumulation on SiC surface by thermal radiation mirroring and raise Si partial pressure near surface by confining the sublimated Si atoms between SiC substrate and Mo-plate, which would be the essential contributors of crystallinity enhancementclose0

Non-Local Signal in Quasi-2DEG of LAO/STO

Electron gas arizen at the insulating oxide interfaces exhibits high electron mobility, tunable carrier densities and related unique behaviors such as coexistence of superconductivity and ferromagnetism, Kondo resistance, etc. Itinerant electrons at the oxide hetero-interface are predicted to have long spin diffusion length, while they are under the relatively strong Rashba-type spin orbit coupling due to inversion symmetry breaking. We studied non-local spin signal induced by spin orbit coupling with additional gate-controlled Rashba field in quasi-2DEG of LaAlO3/SrTiO (LAO/STO) interface. We fabricated simple hall-bar like geometry to measure non-local signal with the variation of channel length (2 ??? 10??m). Cleaned sample was patterned using e-beam lithography and reactive ion etching followed by oxygen treatment to anneal out oxygen vacancies. When an electric current flows one line of the hall bar structure, spin orbit coupling will induce the current flow away from the source current channel via spin hall and inverse spin hall effects. The non-local signals were studied under different angles of magnetic field and the variation of applied gate voltage

Gate tunable spin exchange interaction and inversion of magnetoresistance in ferromagnetic ZnO nanowire

Tuning magnetism in diluted magnetic semiconductor (DMS) is one of the central issue to the development of future spintronic device applications. Particularly, realizing such control in nanostructure has received growing attention. Here, we report the dramatic change of MR in ferromagnetic ZnO nanowire with varied gate voltages (+50 V to -40 V) at different temperatures (2 K to 50 K). The MR signal was greatly influenced by the gate voltage induced carrier concentrations which results the inversion of MR from positive to negative sign while pertaining the coexistence of both parts before inversion in the range of -2T to 2T. The origin of negative MR is mainly due to spin scattering while the positive one is due to a field induced change in relative populations of conduction bands with different conductivities. The extracted spin exchange related parameter was well tuned with the varied gate voltages at different temperatures. More importantly this type of gate tuning of spin exchange interactions in ferromagnetic single ZnO nanowire is well suitable for future spintronic device applications

Gate-Tunable Spin Exchange Interactions and Inversion of Magnetoresistance in Single Ferromagnetic ZnO Nanowires

Electrical control of ferromagnetism in semiconductor nanostructures offers the promise of nonvolatile functionality in future semiconductor spintronics. Here, we demonstrate a dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). 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 to +50 V, the MRs estimated at 2 K and 2 T 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.clos