27 research outputs found
Light-induced magnetization reversal of high-anisotropy TbCo alloy films
Magnetization reversal using circularly polarized light provides a new way to
control magnetization without any external magnetic field and has the potential
to revolutionize magnetic data storage. However, in order to reach ultra-high
density data storage, high anisotropy media providing thermal stability are
needed. Here, we evidence all-optical magnetization switching for different
TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching
for films with anisotropy fields reaching 6 T corresponding to anisotropy
constants of 3x106 ergs/cm3. Optical magnetization switching is observed only
for alloys which compensation temperature can be reached through sample
heating
Effects of rf Current on Spin Transfer Torque Induced Dynamics
The impact of radiofrequency (rf) currents on the direct current (dc) driven
switching dynamics in current-perpendicular-to-plane nanoscale spin valves is
demonstrated. The rf currents dramatically alter the dc driven free layer
magnetization reversal dynamics as well as the dc switching level. This occurs
when the frequency of the rf current is tuned to a frequency range around the
dc driven magnetization precession frequencies. For these frequencies,
interactions between the dc driven precession and the injected rf induce
frequency locking and frequency pulling effects that lead to a measurable
dependence of the critical switching current on the frequency of the injected
rf. Based on macrospin simulations, including dc as well as rf spin torque
currents, we explain the origin of the observed effects.Comment: 5 pages, 4 figure
Spin-Torque-Induced Rotational Dynamics of a Magnetic Vortex Dipole
We study, both experimentally and by numerical modeling, the magnetic
dynamics that can be excited in a magnetic thin-film nanopillar device using
the spin torque from a spatially localized current injected via a
10s-of-nm-diameter aperture. The current-driven magnetic dynamics can produce
large amplitude microwave emission at zero magnetic field, with a frequency
well below that of the uniform ferromagnetic resonance mode. Micromagnetic
simulations indicate that the physical origin of this efficient microwave
nano-oscillator is the nucleation and subsequent steady-state rotational
dynamics of a magnetic vortex dipole driven by the localized spin torque. These
results show this novel implementation of a spintronic nano-oscillator is a
promising candidate for microwave technology applications.Comment: 19 pages, 4 figures
Time-Resolved Spin Torque Switching and Enhanced Damping in Py/Cu/Py Spin-Valve Nanopillars
We report time-resolved measurements of current-induced reversal of a free
magnetic layer in Py/Cu/Py elliptical nanopillars at temperatures T = 4.2 K to
160 K. Comparison of the data to Landau-Lifshitz-Gilbert macrospin simulations
of the free layer switching yields numerical values for the spin torque and the
Gilbert damping parameters as functions of T. The damping is strongly
T-dependent, which we attribute to the antiferromagnetic pinning behavior of a
thin permalloy oxide layer around the perimeter of the free layer. This
adventitious antiferromagnetic pinning layer can have a major impact on spin
torque phenomena.Comment: 5 pages, 4 figure
Electrical Switching Dynamics in Circular and Rectangular Ge2Sb2Te5 Nanopillar Phase Change Memory Devices
We have measured the critical phase change conditions induced by electrical
pulses in Ge2Sb2Te5 nanopillar phase change memory devices by constructing a
comprehensive resistance map as a function of pulse parameters (width,
amplitude and trailing edge). Our measurements reveal that the heating scheme
and the details of the contact geometry play the dominant role in determining
the final phase composition of the device such that a non-uniform heating
scheme promotes partial amorphization/crystallization for a wide range of pulse
parameters enabling multiple resistance levels for data storage applications.
Furthermore we find that fluctuations in the snap-back voltage and set/reset
resistances in repeated switching experiments are related to the details of the
current distribution such that a uniform current injection geometry (i.e.
circular contact) favors more reproducible switching parameters. This shows
that possible geometrical defects in nanoscale phase change memory devices may
play an essential role in the performance of the smallest possible devices
through modification of the exact current distribution in the active
chalcogenide layer. We present a three-dimensional finite element model of the
electro-thermal physics to provide insights into the underlying physical
mechanisms of the switching dynamics as well as to quantitatively account for
the scaling behaviour of the switching currents in both circular and
rectangular contact geometries. The calculated temporal evolution of the heat
distribution within the pulse duration shows distinct features in rectangular
contacts providing evidence for locally hot spots at the sharp corners of the
current injection site due to current crowding effects leading to the observed
behaviour
Strong linewidth variation for spin-torque nano-oscillators as a function of in-plane magnetic field angle
We measure the microwave signals produced by spin-torque-driven magnetization
dynamics in patterned magnetic multilayer devices at room temperature, as a
function of the angle of a magnetic field applied in the sample plane. We find
strong variations in the frequency linewidth of the signals, with a decrease by
more than a factor of 20 as the field is rotated from the magnetic easy axis to
the in-plane hard axis. Based on micromagnetic simulations, we identify these
variations as due to a transition from spatially incoherent to coherent
precession.Comment: 15 pages, 5 figure
Spin-Torque Ferromagnetic Resonance Measurements of Damping in Nanomagnets
We measure the magnetic damping parameter a in thin film CoFeB and permalloy
(Py) nanomagnets at room temperature using ferromagnetic resonance driven by
microwave frequency spin-transfer torque. We obtain and , values comparable to measurements for
extended thin films, but significantly less than the effective damping
determined previously for similar nanomagnets by fits to time-domain studies of
large-angle magnetic excitations and magnetic reversal. The greater damping
found for the large amplitude nanomagnet dynamics is attributed to the
nonlinear excitation of non-uniform magnetic modes.Comment: 13 pages, 2 figure
Magnetic vortex oscillator driven by dc spin-polarized current
Transfer of angular momentum from a spin-polarized current to a ferromagnet
provides an efficient means to control the dynamics of nanomagnets. A peculiar
consequence of this spin-torque, the ability to induce persistent oscillations
of a nanomagnet by applying a dc current, has previously been reported only for
spatially uniform nanomagnets. Here we demonstrate that a quintessentially
nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale
spin valve structure, can be excited into persistent microwave-frequency
oscillations by a spin-polarized dc current. Comparison to micromagnetic
simulations leads to identification of the oscillations with a precession of
the vortex core. The oscillations, which can be obtained in essentially zero
magnetic field, exhibit linewidths that can be narrower than 300 kHz, making
these highly compact spin-torque vortex oscillator devices potential candidates
for microwave signal-processing applications, and a powerful new tool for
fundamental studies of vortex dynamics in magnetic nanostructures.Comment: 14 pages, 4 figure