Material and Structural Enhancements to Spin Transfer Phenomena in Nanopillar Spin-Valve Devices

Abstract

This thesis focuses on work performed to fabricate spin valve nanopillar devices with low spin transfer reversal currents suitable for magnetic memory applications. Fabricating nanopillars into small area ellipses with low saturation magnetization ferromagnets is shown to be an effective strategy for reducing reversal currents while maintaining the thermal stability of the nanomagnet. Pulsed current switching experiments performed on devices with a 4.5 nm thick permalloy free layer show switching current amplitudes ranging from 0.4 mA for a 100 ns pulse to 2 mA for a 1 ns pulse.

I have also examined the role that micromagnetic effects can play in spin transfer reversal processes. Using micromagnetic simulations, a spatially non-uniform spin current with a component polarized partially out of the plane is shown to enhance the spin-torque efficiency acting upon a reversing nanomagnet. I verified this enhancement experimentally in devices with a tapered nanopillar geometry that generates a spin current polarized partially out of plane.

The micromagnetic configurations induced in these tapered nanopillars are also conducive to exciting spin torque driven magnetization oscillations in the absence of an external magnetic field. In addition, by using a small hard axis field the frequencies of oscillations excited in both layers can be tuned such that phase locking occurs between the free and reference layer mediated by spin polarized currents interacting between the layers. This locking phenomenon is character- ized by measured RF voltage signals with large integrated powers and extremely narrow linewidth on the order of 1 HZ.

Finally, I have described a fabrication process for patterning a nanopillar struc- ture with a third contact made to any point within a thin-film multilayer stack, providing the means to apply independent electrical biases to two separate parts of the structure. Here, I have demonstrated a joint magnetic spin valve/tunnel junction structure sharing a common free layer nanomagnet contacted by this third electrode. This three-terminal structure provides a strategy for developing spin-torque magnetic random access memory (ST-RAM) cells which avoids the need to apply large voltages across a magnetic tunnel junction during the writing step, while retaining the benefits of a high-impedance magnetic tunnel junction for read-out.