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Microwave-Frequency Characterization of Spin Transfer and Individual Nanomagnets

Author
Sankey, Jack
Abstract
This dissertation explores the interactions between spin-polarized currents and
individual nanoscale magnets, focusing on the microwave-frequency magnetization
dynamics these currents can excite. Our devices consist of two magnetic films (2-40
nm) separated by a nonmagnetic spacer (5-10 nm Cu or 1.25 nm MgO), patterned
into a "nanopillar" of elliptical cross-section ~100 nm in diameter. One magnetic
layer (a thicker or exchange-biased "fixed" layer) polarizes electron currents
that then apply a spin transfer torque to the other "free" layer. We have
developed several high-frequency techniques in which we excite magnetic dynamics
with spin-polarized currents and detect the corresponding magnetoresistance
oscillations R(t). By applying a direct current I, we can excite both small-angle
and new types of large-angle spontaneous magnetic precession of the free layer,
inducing a microwave voltage V(t) = IR(t) across the junction that we measure
with a spectrum analyzer. By studying the linewidths of the corresponding spectral
peaks as a function of bias and temperature, we find the oscillation coherence
time (related to the inverse linewidth) is limited by thermal fluctuations: deflections
along the precession trajectory for T < 100 K, and thermally-activated mode
hopping for T > 100 K. We have also developed a new form of ferromagnetic
resonance (FMR) in which we use microwave-frequency spin currents to excite dynamics,
and a resonant (DC) mixing voltage to measure the response. With this
technique we can directly probe the magnetic damping in both layers, identify the
dynamical modes observed in the DC-driven experiment, observe phase locking
with these modes, and even probe the physical form of the spin transfer torque.
For metallic devices we find the torque is always confined to the plane of the layers?
magnetizations, while for (MgO) tunnel junctions we find a new component of the
torque perpendicular to this plane, appearing at higher bias voltages. This new
FMR technique should be able to probe much smaller devices still, enabling new
fundamental studies of even smaller magnetic samples, someday approaching the
molecular limit.
Sponsorship
DARPA through Motorola,
the Office of Naval Research,
the Army Research Office,
NSF (DMR-0605742),
NSF/NSEC program through the Cornell Center for Nanoscale Systems,
NSF support through use of the Cornell Nanofabrication Facility/
NNIN and the Cornell Center for Materials Research facilities.
Date Issued
2007-05-15Subject
Spin; Transfer; Spin Transfer; Magnetism; Nanomagnet; ferromagnetic resonance; precession
Has other format(s)
bibid: 6476318
Type
dissertation or thesis