27 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
Bimodal switching field distributions in all-perpendicular spin-valve nanopillars
Switching field measurements of the free layer element of 75 nm diameter
spin-valve nanopillars reveal a bimodal distribution of switching fields at low
temperatures (below 100 K). This result is inconsistent with a model of thermal
activation over a single perpendicular anisotropy barrier. The correlation
between antiparallel to parallel and parallel to antiparallel switching fields
increases to nearly 50% at low temperatures. This reflects random fluctuation
of the shift of the free layer hysteresis loop between two different
magnitudes, which may originate from changes in the dipole field from the
polarizing layer. The magnitude of the loop shift changes by 25% and is
correlated to transitions of the spin-valve into an antiparallel configuration.Comment: 3 pages, 4 figures. Submitted to JAP for 58th MMM Proceeding
Spin transfer switching of spin valve nanopillars using nanosecond pulsed currents
Spin valve nanopillars are reversed via the mechanism of spin momentum
transfer using current pulses applied perpendicular to the film plane of the
device. The applied pulses were varied in amplitude from 1.8 mA to 7.8 mA, and
varied in duration within the range of 100 ps to 200 ns. The probability of
device reversal is measured as a function of the pulse duration for each pulse
amplitude. The reciprocal pulse duration required for 95% reversal probability
is linearly related to the pulse current amplitude for currents exceeding 1.9
mA. For this device, 1.9 mA marks the crossover between dynamic reversal at
larger currents and reversal by thermal activation for smaller currents
Parametric resonance of magnetization excited by electric field
Manipulation of magnetization by electric field is a central goal of
spintronics because it enables energy-efficient operation of spin-based
devices. Spin wave devices are promising candidates for low-power information
processing but a method for energy-efficient excitation of short-wavelength
spin waves has been lacking. Here we show that spin waves in nanoscale magnetic
tunnel junctions can be generated via parametric resonance induced by electric
field. Parametric excitation of magnetization is a versatile method of
short-wavelength spin wave generation, and thus our results pave the way
towards energy-efficient nanomagnonic devices
Magnetization reversal driven by low dimensional chaos in a nanoscale ferromagnet
Energy-efficient switching of magnetization is a central problem in
nonvolatile magnetic storage and magnetic neuromorphic computing. In the past
two decades, several efficient methods of magnetic switching were demonstrated
including spin torque, magneto-electric, and microwave-assisted switching
mechanisms. Here we report the discovery of a new mechanism giving rise to
magnetic switching. We experimentally show that low-dimensional magnetic chaos
induced by alternating spin torque can strongly increase the rate of
thermally-activated magnetic switching in a nanoscale ferromagnet. This
mechanism exhibits a well-pronounced threshold character in spin torque
amplitude and its efficiency increases with decreasing spin torque frequency.
We present analytical and numerical calculations that quantitatively explain
these experimental findings and reveal the key role played by low-dimensional
magnetic chaos near saddle equilibria in enhancement of the switching rate. Our
work unveils an important interplay between chaos and stochasticity in the
energy assisted switching of magnetic nanosystems and paves the way towards
improved energy efficiency of spin torque memory and logic
Asymmetric switching behavior in perpendicularly magnetized spin-valve nanopillars due to the polarizer dipole field
We report the free layer switching field distributions of spin-valve
nanopillars with perpendicular magnetization. While the distributions are
consistent with a thermal activation model, they show a strong asymmetry
between the parallel to antiparallel and the reverse transition, with energy
barriers more than 50% higher for the parallel to antiparallel transitions. The
inhomogeneous dipolar field from the polarizer is demonstrated to be at the
origin of this symmetry breaking. Interestingly, the symmetry is restored for
devices with a lithographically defined notch pair removed from the midpoint of
the pillar cross-section along the ellipse long axis. These results have
important implications for the thermal stability of perpendicular magnetized
MRAM bit cells.Comment: Submitted to Applied Physics Letters on November 4, 2011. Consists of
4 pages, 3 figure
Ultralow-current-density and bias-field-free spin-transfer nano-oscillator
The spin-transfer nano-oscillator (STNO) offers the possibility of using the
transfer of spin angular momentum via spin-polarized currents to generate
microwave signals. However, at present STNO microwave emission mainly relies on
both large drive currents and external magnetic fields. These issues hinder the
implementation of STNOs for practical applications in terms of power
dissipation and size. Here, we report microwave measurements on STNOs built
with MgO-based magnetic tunnel junctions having a planar polarizer and a
perpendicular free layer, where microwave emission with large output power,
excited at ultralow current densities, and in the absence of any bias magnetic
fields is observed. The measured critical current density is over one order of
magnitude smaller than previously reported. These results suggest the
possibility of improved integration of STNOs with complementary
metal-oxide-semiconductor technology, and could represent a new route for the
development of the next-generation of on-chip oscillators.Comment: 18 pages, 4 figure