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

Author
Braganca, Patrick M
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.
Description
Robert Buhrman - Thesis Advisor,
Dan Ralph - Committee member,
Bruce van Dover - Committee member,
Date Issued
2008-08-26Subject
Nanomagnetics; Spin transfer; Spin torque
Type
dissertation or thesis