79 research outputs found
A Frequency-Controlled Magnetic Vortex Memory
Using the ultra low damping NiMnSb half-Heusler alloy patterned into
vortex-state magnetic nano-dots, we demonstrate a new concept of non-volatile
memory controlled by the frequency. A perpendicular bias magnetic field is used
to split the frequency of the vortex core gyrotropic rotation into two distinct
frequencies, depending on the sign of the vortex core polarity inside
the dot. A magnetic resonance force microscope and microwave pulses applied at
one of these two resonant frequencies allow for local and deterministic
addressing of binary information (core polarity)
Dream Self-Reflectiveness as a Learned Cognitive Skill
Many prominent researchers subscribe to the notion that dreaming is cognitively deficient relative to normal waking consciousness (Foulkes,1983; Hartmann, 1973; Koukkou & Lehman, 1983). Dreams are perceived as massively non-reflective and single-minded as evidenced by their apparent lack of imagination, lack of lucidity (awareness of dreaming while dreaming), and tendency to be forgotten (Rechtschaffen, 1978). The notion of dream ‘isolation’ from other systems of consciousness has been posited by Rechtachaffen as an inescapable conclusion once these characteristics of dreaming have been understood
Bistability of vortex core dynamics in a single perpendicularly magnetized nano-disk
Microwave spectroscopy of individual vortex-state magnetic nano-disks in a
perpendicular bias magnetic field, , is performed using a magnetic resonance
force microscope (MRFM). It reveals the splitting induced by on the
gyrotropic frequency of the vortex core rotation related to the existence of
the two stable polarities of the core. This splitting enables spectroscopic
detection of the core polarity. The bistability extends up to a large negative
(antiparallel to the core) value of the bias magnetic field , at which the
core polarity is reversed. The difference between the frequencies of the two
stable rotational modes corresponding to each core polarity is proportional to
and to the ratio of the disk thickness to its radius. Simple analytic
theory in combination with micromagnetic simulations give quantitative
description of the observed bistable dynamics.Comment: 4 pages, 3 figures, 1 table, 16 references. Submitted to Physical
Review Letters on December 19th, 200
Identification and selection rules of the spin-wave eigen-modes in a normally magnetized nano-pillar
We report on a spectroscopic study of the spin-wave eigen-modes inside an
individual normally magnetized two layers circular nano-pillar
(PermalloyCopperPermalloy) by means of a Magnetic Resonance Force
Microscope (MRFM). We demonstrate that the observed spin-wave spectrum
critically depends on the method of excitation. While the spatially uniform
radio-frequency (RF) magnetic field excites only the axially symmetric modes
having azimuthal index , the RF current flowing through the
nano-pillar, creating a circular RF Oersted field, excites only the modes
having azimuthal index . Breaking the axial symmetry of the
nano-pillar, either by tilting the bias magnetic field or by making the pillar
shape elliptical, mixes different -index symmetries, which can be excited
simultaneously by the RF current. Experimental spectra are compared to
theoretical prediction using both analytical and numerical calculations. An
analysis of the influence of the static and dynamic dipolar coupling between
the nano-pillar magnetic layers on the mode spectrum is performed
Universal Vectorial and Ultrasensitive Nanomechanical Force Field Sensor
Miniaturization of force probes into nanomechanical oscillators enables
ultrasensitive investigations of forces on dimensions smaller than their
characteristic length scale. Meanwhile it also unravels the force field
vectorial character and how its topology impacts the measurement. Here we
expose an ultrasensitive method to image 2D vectorial force fields by
optomechanically following the bidimensional Brownian motion of a singly
clamped nanowire. This novel approach relies on angular and spectral tomography
of its quasi frequency-degenerated transverse mechanical polarizations:
immersing the nanoresonator in a vectorial force field does not only shift its
eigenfrequencies but also rotate eigenmodes orientation as a nano-compass. This
universal method is employed to map a tunable electrostatic force field whose
spatial gradients can even take precedence over the intrinsic nanowire
properties. Enabling vectorial force fields imaging with demonstrated
sensitivities of attonewton variations over the nanoprobe Brownian trajectory
will have strong impact on scientific exploration at the nanoscale
Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme
Spin-polarised radio-frequency currents, whose frequency is equal to that of
the gyrotropic mode, will cause an excitation of the core of a magnetic vortex
confined in a magnetic tunnel junction. When the excitation radius of the
vortex core is greater than that of the junction radius, vortex core expulsion
is observed, leading to a large change in resistance, as the layer enters a
predominantly uniform magnetisation state. Unlike the conventional spin-torque
diode effect, this highly tunable resonant effect will generate a voltage which
does not decrease as a function of rf power, and has the potential to form the
basis of a new generation of tunable nanoscale radio-frequency detectors
Magnetic Vortex Core Reversal by Excitation of Spin Waves
Micron-sized magnetic platelets in the flux closed vortex state are
characterized by an in-plane curling magnetization and a nanometer-sized
perpendicularly magnetized vortex core. Having the simplest non-trivial
configuration, these objects are of general interest to micromagnetics and may
offer new routes for spintronics applications. Essential progress in the
understanding of nonlinear vortex dynamics was achieved when low-field core
toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was
established. At frequencies more than an order of magnitude higher vortex state
structures possess spin wave eigenmodes arising from the magneto-static
interaction. Here we demonstrate experimentally that the unidirectional vortex
core reversal process also occurs when such azimuthal modes are excited. These
results are confirmed by micromagnetic simulations which clearly show the
selection rules for this novel reversal mechanism. Our analysis reveals that
for spin wave excitation the concept of a critical velocity as the switching
condition has to be modified.Comment: Minor corrections and polishing of previous versio
Optimal control of vortex core polarity by resonant microwave pulses
In a vortex-state magnetic nano-disk, the static magnetization is curling in
the plane, except in the core region where it is pointing out-of-plane, either
up or down leading to two possible stable states of opposite core polarity p.
Dynamical reversal of p by large amplitude motion of the vortex core has
recently been demonstrated experimentally,raising fundamental interest for
potential application in magnetic storage devices. Here we demonstrate coherent
control of p by single and double microwave pulse sequences, taking advantage
of the resonant vortex dynamics in a perpendicular bias magnetic field.
Optimization of the microwave pulse duration required to switch p allows to
experimentally infer the characteristic decay time of the vortex core in the
large oscillation regime. It is found to be more than twice shorter than in the
small oscillation regime, raising the fundamental question of the non-linear
behaviour of magnetic dissipation
Identification and selection rules of the spin-wave eigenmodes in a normally magnetized nanopillar
We report on a spectroscopic study of the spin-wave eigenmodes inside an individual normally magnetized two-layer circular nanopillar (permalloy|copper|permalloy) by means of a magnetic resonance force microscope. We demonstrate that the observed spin-wave spectrum critically depends on the method of excitation. While the spatially uniform radio-frequency (rf) magnetic field excites only the axially symmetric modes having azimuthal index =0, the rf current flowing through the nanopillar, creating a circular rf Oersted field, excites only the modes having azimuthal index =+1. Breaking the axial symmetry of the nanopillar, either by tilting the bias magnetic field or by making the pillar shape elliptical, mixes different index symmetries, which can be excited simultaneously by the rf current. Experimental spectra are compared to theoretical prediction using both analytical and numerical calculations. An analysis of the influence of the static and dynamic dipolar coupling between the nanopillar magnetic layers on the mode spectrum is performed. © 2011 American Physical Society
Remodelling of Cortical Actin Where Lytic Granules Dock at Natural Killer Cell Immune Synapses Revealed by Super-Resolution Microscopy
Super-resolution 3D imaging reveals remodeling of the cortical actin meshwork at the natural killer cell immune synapse, which is likely to be important for secretion of lytic granules
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