44 research outputs found
Time Domain Mapping of Spin Torque Oscillator Effective Energy
Stochastic dynamics of spin torque oscillators (STOs) can be described in
terms of magnetization drift and diffusion over a current-dependent effective
energy surface given by the Fokker-Planck equation. Here we present a method
that directly probes this effective energy surface via time-resolved
measurements of the microwave voltage generated by a STO. We show that the
effective energy approach provides a simple recipe for predicting spectral line
widths and line shapes near the generation threshold. Our time domain technique
also accurately measures the field-like component of spin torque in a wide
range of the voltage bias values.Comment: 5 pages, 3 figures. Supplement included: 7 pages, 6 figure
Angular Dependence of the Superconducting Transition Temperature in Ferromagnet-Superconductor-Ferromagnet Trilayers
The superconducting transition temperature, , of a ferromagnet (F) -
superconductor (S) - ferromagnet trilayer depends on the mutual orientation of
the magnetic moments of the F layers. This effect has been previously observed
in F/S/F systems as a difference between parallel and antiparallel
configurations of the F layers. Here we report measurements of in
CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments
of the CuNi ferromagnets. The observed angular dependence of is in
qualitative agreement with a F/S proximity theory that accounts for the odd
triplet component of the condensate predicted to arise for non-collinear
orientation of the magnetic moments of the F layers.Comment: 4 + \epsilon pages including 4 figures. To appear in Phys. Rev. Let
Low Power Microwave Signal Detection With a Spin-Torque Nano-Oscillator in the Active Self-Oscillating Regime
A spin-torque nano-oscillator (STNO) driven by a ramped bias current can
perform spectrum analysis quickly over a wide frequency bandwidth. The STNO
spectrum analyzer operates by injection locking to external microwave signals
and produces an output DC voltage that temporally encodes the
input spectrum. We found, via numerical analysis with a macrospin
approximation, that an STNO is able to scan a bandwidth in less
than (scanning rate exceeds ). In contrast to
conventional quadratic microwave detectors, the output voltage of the STNO
analyzer is proportional to the amplitude of the input microwave signal with sensitivity . The
minimum detectable signal of the analyzer depends on the scanning rate and,
at low , is about .Comment: 5 pages, 5 figure
Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques
Spin torque from spin current applied to a nanoscale region of a ferromagnet
can act as negative magnetic damping and thereby excite self-oscillations of
its magnetization. In contrast, spin torque uniformly applied to the
magnetization of an extended ferromagnetic film does not generate
self-oscillatory magnetic dynamics but leads to reduction of the saturation
magnetization. Here we report studies of the effect of spin torque on a system
of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent
self-oscillations of magnetization in a ferromagnetic nanowire serving as the
active region of a spin torque oscillator driven by spin orbit torques. Our
work demonstrates that magnetization self-oscillations can be excited in a
one-dimensional magnetic system and that dimensions of the active region of
spin torque oscillators can be extended beyond the nanometer length scale.Comment: The link to the published version is
http://www.nature.com/ncomms/2014/141205/ncomms6616/full/ncomms6616.htm
Easy-plane spin Hall oscillator
Spin Hall oscillators (SHOs) based on bilayers of a ferromagnet (FM) and a
non-magnetic heavy metal (HM) are electrically tunable nanoscale microwave
signal generators. Achieving high output power in SHOs requires driving
large-amplitude magnetization dynamics by a direct spin Hall current. The
maximum possible amplitude of such oscillations with the precession cone angle
nearing is predicted for FM layers with easy-plane magnetic
anisotropy and spin Hall current polarization perpendicular to the easy plane.
While many FMs exhibit natural easy-plane anisotropy in the FM film plane, the
spin Hall current in a HM|FM bilayer is polarized in this plane and thus cannot
drive large-amplitude magneto-dynamics. Here we present a new type of SHO
engineered to have the easy-plane anisotropy oriented normal to the film plane,
enabling large-amplitude easy-plane dynamics driven by spin Hall current. Our
experiments and micromagnetic simulations demonstrate that the desired
easy-plane anisotropy can be achieved by tuning the magnetic shape anisotropy
and perpendicular magnetic anisotropy in a nanowire SHO, leading to a
significant enhancement of the generated microwave power. The easy-plane SHO
experimentally demonstrated here is an ideal candidate for realization of a
spintronic spiking neuron. Our results provide a new approach to design of
high-power SHOs for wireless communications, neuromorphic computing, and
microwave assisted magnetic recording.Comment: Main: 12 pages, 6 figures. Supplementary Material: 5 pages, 6 figure
Angular dependence of superconductivity in superconductor / spin valve heterostructures
We report measurements of the superconducting transition temperature, ,
in CoO/Co/Cu/Co/Nb multilayers as a function of the angle between the
magnetic moments of the Co layers. Our measurements reveal that
is a nonmonotonic function, with a minimum near . Numerical
self-consistent solutions of the Bogoliubov - de Gennes equations
quantitatively and accurately describe the behavior of as a function of
and layer thicknesses in these superconductor / spin-valve
heterostructures. We show that experimental data and theoretical evidence agree
in relating to enhanced penetration of the triplet component of
the condensate into the Co/Cu/Co spin valve in the maximally noncollinear
magnetic configuration.Comment: 9 pages, 9 figure