Condensed-Matter Physics & Materials Science Seminar

Spintronics utilizes the quantum spins of electrons for information storage and processing. A spin current, essential to various spintronic functionalities, is a flow of spin angular momentum carried by moving electrons. A pure spin current, in contrast to a spin-polarized charge current, can be generated in a situation where electrons with opposite spins move in opposite directions. With a pure spin current, the flow of the spin angular momentum can be substantial while the net charge current is zero. Adverse effects to nanoscale electronic devices induced by a charge current, such as electromigration and Joule heating, can be effectively tamed with a pure spin current. In this presentation, I will present two recently observed phenomena in metallic nanoscale nonlocal spin valves (NLSV), where a pure spin current can be explored. Firstly, we utilize the pure spin current to switch the magnetization of a nanomagnet. The magnetization reversal is accomplished by the transfer of spin angular momentum from the spin current to the magnetization. This effect, known as spin-transfer, has been explored extensively with polarized charge currents, but not as much with pure spin currents. We demonstrate the spin-transfer with modest injection currents in a wide temperature range between 4 K and 200 K. Evidence for spin-transfer-induced spin dynamics is also observed. Secondly, we observed a large and inverted spin accumulation signal in a set of NLSV structures, in which the spin-detector junctions are actually vacuum break-junctions formed by static discharge. The effective detection spin polarization is large (>80%) but with a negative sign. A theoretical model that emphasizes on the spin-charge coupling across the low-conductance break-junction is used to interpret the result.