UC Riverside

This dissertation consists of 3 different research projects and the first major part is devoted to exploring the spin absorption effect through a transparent Ferro magnet junction on graphene and second major part investigates an advanced nanofabrication technique: multiple angle deposition for the realization of spin transfer torque in graphene lateral non-local spin valve devices in which pure spin current can be produced. A small portion of this thesis is also devoted to the characterization of top gated transistors fabricated on CVD grown and exfoliated MoS2 film using HfO2 as a dielectric.

Spin absorption is of current interest for the realization of current induced magnetization reversal of nano-scale magnets which is the key for future spin transfer torque devices. We studied spin absorption by a Permalloy nanomagnet grown on top of a lateral graphene spin valve channel. A pure spin current is injected into a graphene channel via electrical spin injection through a Co/Al2O3 tunnel barrier junction. The Permalloy island in between the injector and the detector is expected to modulate the spin population at the detector via spin absorption. Depending on the magnetization of the Py island, the spin current can be absorbed differently resulting in a distinct signature of non-local magnetoresistance (MR) from the Py island magnetization and hence a modulation of the spin population. We attribute this effect to a combination of both transverse and longitudinal spin current absorptions which is caused by the micro domain formations within the Perm alloy island and hence a hysteretic behavior in the MR signal. This hysteretic behavior can be due to the magnetization rotation of micro domains which raise the discussion of both lateral and longitudinal spin absorption. It is very important to investigate the anisotropy of the island and correlate the result to the non-local MR measurement. To this end, magnetic force microscopy of the island is needed in order to further confirm the observed non local signal in our devices. This new effect is still being investigated and further studies are underway to unravel the origin of this new effect observed in graphene spin valves.