309 research outputs found
Spin torque driven dynamics of a coupled two layer structure: interplay between conservative and dissipative coupling
In this manuscript the general concepts of spin wave theory are adapted to
the dynamics of a self-polarized system based on two layers coupled via
interlayer exchange (conservative coupling) and mutual spin torque (dissipative
coupling). An analytical description of the non-linear dynamics is proposed and
validated through numerical simulations. In contrast to the single layer model,
the phase equation of the coupled system has a contribution coming from the
dissipative part of the LLGS equation. It is shown that this is a major
contribution to the frequency mandatory to describe well the most basic
features of the dynamics of coupled systems. Using the proposed model a
specific feature of coupled dynamics is addressed: the redshift to blueshift
transition observed in the frequency current dependence of this kind of
exchange coupled systems upon increasing the applied field. It is found that
the blueshift regime can only occur in a region of field where the two linear
eigenmodes contribute equally to the steady state mode (i.e. high mode
hybridization). Finally, a general perturbed Hamiltonian equation for the
coupled system is proposed.Comment: 16 pages, 7 figue
Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions
The relative contributions of in-plane (damping-like) and out-of-plane
(field-like) spin-transfer-torques in the magnetization switching of
out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically
analyzed using the transformed Landau-Lifshitz (LL) equation with the STT
terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics,
the out-of-plane torque influences the precession frequency but it does not
contribute significantly to the STT switching process (in particular to the
switching time and switching current density), which is mostly determined by
the in-plane STT contribution. This conclusion is confirmed by finite
temperature and finite writing pulse macrospin simulations of the current-field
switching diagrams. It contrasts with the case of STT-switching in in-plane
magnetized MTJ in which the field-like term also influences the switching
critical current. This theoretical analysis was successfully applied to the
interpretation of voltage-field STT switching diagrams experimentally measured
on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like
behavior. The physical nonequivalence of Landau and Gilbert dissipation terms
in presence of STT-induced dynamics is also discussed
Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio
We study in-plane magnetic tunnel junctions with additional perpendicular
polarizer for subnanosecond-current-induced switching memories. The
spin-transfer-torque switching dynamics was studied as a function of the cell
aspect ratio both experimentally and by numerical simulations using the
macrospin model. We show that the anisotropy field plays a significant role in
the dynamics, along with the relative amplitude of the two spin-torque
contributions. This was confirmed by micromagnetic simulations. Real-time
measurements of the reversal were performed with samples of low and high aspect
ratio. For low aspect ratios, a precessional motion of the magnetization was
observed and the effect of temperature on the precession coherence was studied.
For high aspect ratios, we observed magnetization reversals in less than 1 ns
for high enough current densities, the final state being controlled by the
current direction in the magnetic tunnel junction cell.Comment: 6 pages, 7 figure
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
Current induced domain wall dynamics in the presence of spin orbit torques
Current induced domain wall (DW) motion in perpendicularly magnetized
nanostripes in the presence of spin orbit torques is studied. We show using
micromagnetic simulations that the direction of the current induced DW motion
and the associated DW velocity depend on the relative values of the field like
torque (FLT) and the Slonczewski like torques (SLT). The results are well
explained by a collective coordinate model which is used to draw a phase
diagram of the DW dynamics as a function of the FLT and the SLT. We show that a
large increase in the DW velocity can be reached by a proper tuning of both
torques.Comment: 9 pages, 3 figure
Dimensionality cross-over in magnetism: from domain walls (2D) to vortices (1D)
Dimensionality cross-over is a classical topic in physics. Surprisingly it
has not been searched in micromagnetism, which deals with objects such as
domain walls (2D) and vortices (1D). We predict by simulation a second-order
transition between these two objects, with the wall length as the Landau
parameter. This was conrmed experimentally based on micron-sized ux-closure
dots
Domain wall tilting in the presence of the Dzyaloshinskii-Moriya interaction in out-of-plane magnetized magnetic nanotracks
We show that the Dzyaloshinskii-Moriya interaction (DMI) can lead to a
tilting of the domain wall (DW) surface in perpendicularly magnetized magnetic
nanotracks when DW dynamics is driven by an easy axis magnetic field or a spin
polarized current. The DW tilting affects the DW dynamics for large DMI and the
tilting relaxation time can be very large as it scales with the square of the
track width. The results are well explained by an analytical model based on a
Lagrangian approach where the DMI and the DW tilting are included. We propose a
simple way to estimate the DMI in a magnetic multilayers by measuring the
dependence of the DW tilt angle on a transverse static magnetic field. Our
results shed light on the current induced DW tilting observed recently in Co/Ni
multilayers with inversion asymmetry, and further support the presence of DMI
in these systems.Comment: 12 pages, 3 figures, 1 Supplementary Material
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