59 research outputs found
Fast computation of magnetostatic fields by Non-uniform Fast Fourier Transforms
The bottleneck of micromagnetic simulations is the computation of the
long-ranged magnetostatic fields. This can be tackled on regular N-node grids
with Fast Fourier Transforms in time N logN, whereas the geometrically more
versatile finite element methods (FEM) are bounded to N^4/3 in the best case.
We report the implementation of a Non-uniform Fast Fourier Transform algorithm
which brings a N logN convergence to FEM, with no loss of accuracy in the
results
Chiral damping of magnetic domain walls
Structural symmetry breaking in magnetic materials is responsible for a
variety of outstanding physical phenomena. Examples range from the existence of
multiferroics, to current induced spin orbit torques (SOT) and the formation of
topological magnetic structures. In this letter we bring into light a novel
effect of the structural inversion asymmetry (SIA): a chiral damping mechanism.
This phenomenon is evidenced by measuring the field driven domain wall (DW)
motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The
difficulty in evidencing the chiral damping is that the ensuing DW dynamics
exhibit identical spatial symmetry to those expected from the
Dzyaloshinskii-Moriya interaction (DMI). Despite this fundamental resemblance,
the two scenarios are differentiated by their time reversal properties: while
DMI is a conservative effect that can be modeled by an effective field, the
chiral damping is purely dissipative and has no influence on the equilibrium
magnetic texture. When the DW motion is modulated by an in-plane magnetic
field, it reveals the structure of the internal fields experienced by the DWs,
allowing to distinguish the physical mechanism. The observation of the chiral
damping, not only enriches the spectrum of physical phenomena engendered by the
SIA, but since it can coexists with DMI it is essential for conceiving DW and
skyrmion devices
Optical Switching in Tb/Co-Multilayer Based Nanoscale Magnetic Tunnel Junctions
Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory
devices. For high-speed and low-power data storage and processing applications,
fast reversal by an ultrashort laser pulse is extremely important. We
demonstrate optical switching of Tb/Comultilayer-based nanoscale MTJs by
combining optical writing and electrical read-out methods. A 90 fs-long laser
pulse switches the magnetization of the storage layer (SL). The change in
magnetoresistance between the SL and a reference layer (RL) is probed
electrically across the tunnel barrier. Single-shot switching is demonstrated
by varying the cell diameter from 300 nm to 20 nm. The anisotropy,
magnetostatic coupling, and switching probability exhibit cell-size dependence.
By suitable association of laser fluence and magnetic field, successive
commutation between high-resistance and low-resistance states is achieved. The
switching dynamics in a continuous film is probed with the magneto-optical Kerr
effect technique. Our experimental findings provide strong support for the
growing interest in ultrafast spintronic devices.Comment: total pages 22, Total figure
Spiking Dynamics in Dual Free Layer Perpendicular Magnetic Tunnel Junctions
Spintronic devices have recently attracted a lot of attention in the field of
unconventional computing due to their non-volatility for short and long term
memory, non-linear fast response and relatively small footprint. Here we report
how voltage driven magnetization dynamics of dual free layer perpendicular
magnetic tunnel junctions enable to emulate spiking neurons in hardware. The
output spiking rate was controlled by varying the dc bias voltage across the
device. The field-free operation of this two terminal device and its robustness
against an externally applied magnetic field make it a suitable candidate to
mimic neuron response in a dense Neural Network (NN). The small energy
consumption of the device (4-16 pJ/spike) and its scalability are important
benefits for embedded applications. This compact perpendicular magnetic tunnel
junction structure could finally bring spiking neural networks (SNN) to
sub-100nm size elements
Gate-Controlled Skyrmion Chirality
Magnetic skyrmions are localized chiral spin textures, which offer great
promise to store and process information at the nanoscale. In the presence of
asymmetric exchange interactions, their chirality, which governs their
dynamics, is generally considered as an intrinsic parameter set during the
sample deposition. In this work, we experimentally demonstrate that this key
parameter can be controlled by a gate voltage. We observed that the
current-induced skyrmion motion can be reversed by the application of a gate
voltage. This local and dynamical reversal of the skyrmion chirality is due to
a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we
attribute to ionic migration of oxygen under gate voltage. Micromagnetic
simulations show that the chirality reversal is a continuous transformation, in
which the skyrmion is conserved. This gate-controlled chirality provides a
local and dynamical degree of freedom, yielding new functionalities to
skyrmion-based logic devices.Comment: 4 figure
Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures
Magnetic skyrmions are chiral spin structures with a whirling configuration.
Their topological properties, nanometer size and the fact that they can be
moved by small current densities have opened a new paradigm for the
manipulation of magnetisation at the nanoscale. To date, chiral skyrmion
structures have been experimentally demonstrated only in bulk materials and in
epitaxial ultrathin films and under external magnetic field or at low
temperature. Here, we report on the observation of stable skyrmions in
sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero
applied magnetic field. We use high lateral resolution X-ray magnetic circular
dichroism microscopy to image their chiral N\'eel internal structure which we
explain as due to the large strength of the Dzyaloshinskii-Moriya interaction
as revealed by spin wave spectroscopy measurements. Our results are
substantiated by micromagnetic simulations and numerical models, which allow
the identification of the physical mechanisms governing the size and stability
of the skyrmions.Comment: Submitted version. Extended version to appear in Nature
Nanotechnolog
Modelling of spintronic devices: from basic operation mechanisms toward optimization
Nowadays the spintronic related phenomena are widely investigated since very promising device concepts have been proposed. Combined experimental and theoretical studies enabled a tremendous evolution of this topic in the case of confined magnetic systems such as thin films, nanopillars or nanowires. The magnetization dynamics inside these magnetic nano-objects must be precisely understood and controlled for ensuring their efficient operation. The view point from micromagnetic modelling will be presented starting with the numerical tools and their use to carry extended numerical studies. Several types of spintronic devices will be addressed combining analytical and numerical modelling intimately related with experimental results
Modelling of spintronic devices: from basic operation mechanisms toward optimization
Nowadays the spintronic related phenomena are widely investigated since very promising device concepts have been proposed. Combined experimental and theoretical studies enabled a tremendous evolution of this topic in the case of confined magnetic systems such as thin films, nanopillars or nanowires. The magnetization dynamics inside these magnetic nano-objects must be precisely understood and controlled for ensuring their efficient operation. The view point from micromagnetic modelling will be presented starting with the numerical tools and their use to carry extended numerical studies. Several types of spintronic devices will be addressed combining analytical and numerical modelling intimately related with experimental results
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