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Spin Torque and Spin-Dependent Transport in Nanoscale Devices


Spin Hall effect in a heavy metal (HM) generates a pure spin current flowing perpendicular to an applied electric charge current. When injected into a ferromagnet (FM), this pure spin current can act as negative magnetic damping thereby exciting self-oscillations of magnetization. In a simple HM/FM bilayer geometry, the current-driven self-oscillations of magnetization result in a microwave voltage generation due to anisotropic magneto-resistance (AMR) of the FM. Since AMR in thin films of typical FM materials such as Permalloy (Py) is relatively small, the output microwave signal generated by the HM/FM bilayer spin Hall oscillators (SHOs) is typically limited to ~ 0.1 nW. In this thesis, a new type of SHO by replacing FM layer with spin valve multilayers. In this type of devices, the microwave power generation relies on current-in-plane (CIP) giant magneto-resistance (GMR) instead of AMR. Since the magnitude GMR typically exceeds that of AMR, this new type of SHO can generate significantly higher power than the AMR-based SHOs. The maximum microwave power generated by the device exceeds 1 nW, which is over an order of magnitude higher than that in HM/FM bilayer SHO nanowire devices.

Spin Transfer Torque Random Access Memory (STT-RAM) is a promising non-volatile memory technology that offers scalable area, fast operation, and low power consumption advantages over traditional SRAM, DRAM, and flash memories. Performance of the STT-RAM sensitively depends on magnetic anisotropy and properties of the magnetic tunnel junctions (MTJs), which is the most essential part in STT-RAM.

We present here the results on the effect of ionizing radiation on perpendicular-anisotropy MTJs (PMTJs). These samples were exposed to both gamma radiation and a mix of gamma and thermal neutron, using the TRIGA reactor. The studies were on TMR, the electrical transport measurements taken on the MTJ nanopillars, both before and after the irradiation. Our results match the previous study\cite{Jander2012}, in which no explicit changes on TMR curves of MTJs after the gamma and neutron irradiation. We also investigated the radiation effect on current switching TMR loop of PMTJs that no one reported before. To be confirmed robustness of MTJs statistically, more than 150 devices were investigated in this study.

Non-local lateral devices have been extensively employed for studies of spin-dependent transport in a wide range of non-magnetic (NM) metals and semiconductors. In these devices, pure spin currents in non-magnetic channel materials can be generated and their propagation and decay can be electrically detected. We have successfully observed a non-local spin valve signal in a topological insulator (TI) channel material Sb$_2$Te$_3$. The characteristic property of TI, spin-momentum locking (SML), was identified. The spins of the TI Dirac surface state lies in-plane, and is locked at right angles to the electrons' momentum.

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