105 research outputs found
Magnetic non-contact friction from domain wall dynamics actuated by oscillatory mechanical motion
Magnetic friction is a form of non-contact friction arising from the
dissipation of energy in a magnet due to spin reorientation in a magnetic
field. In this paper we study magnetic friction in the context of
micromagnetics, using our recent implementation of smooth spring-driven motion
[Phys. Rev. E. 97, 053301 (2018)] to simulate ring-down measurements in two
setups where domain wall dynamics is induced by mechanical motion. These
include a single thin film with a domain wall in an external field and a setup
mimicking a magnetic cantilever tip and substrate, in which the two magnets
interact through dipolar interactions. We investigate how various micromagnetic
parameters influence the domain wall dynamics actuated by the oscillatory
spring-driven mechanical motion and the resulting damping coefficient. Our
simulations show that the magnitude of magnetic friction can be comparable to
other forms of non-contact friction. For oscillation frequencies lower than
those inducing excitations of the internal structure of the domain walls, the
damping coefficient is found to be independent of frequency. Hence, our results
obtained in the frequency range from 8 to 112 MHz are expected to be relevant
also for typical experimental setups operating in the 100 kHz range.Comment: 19 pages, 8 figure
Domain walls within domain walls in wide ferromagnetic strips
We carry out large-scale micromagnetic simulations which demonstrate that due
to topological constraints, internal domain walls (Bloch lines) within extended
domain walls are more robust than domain walls in nanowires. Thus, the
possibility of spintronics applications based on their motion channeled along
domain walls emerges. Internal domain walls are nucleated within domain walls
in perpendicularly magnetized media concurrent with a Walker breakdown-like
abrupt reduction of the domain wall velocity above a threshold driving force,
and may also be generated within pinned, localized domain walls. We observe
fast field and current driven internal domain wall dynamics without a Walker
breakdown along pinned domain walls, originating from topological protection of
the internal domain wall structure due to the surrounding out-of-plane domains.Comment: 5 pages, 6 figure
Universality classes and crossover scaling of Barkhausen noise in thin films
We study the dynamics of head-to-head domain walls separating in-plane
domains in a disordered ferromagnetic thin film. The competition between the
domain wall surface tension and dipolar interactions induces a crossover
between a rough domain wall phase at short length-scales and a large-scale
phase where the walls display a zigzag morphology. The two phases are
characterized by different critical exponents for Barkhausen avalanche dynamics
that are in quantitative agreement with experimental measurements on MnAs thin
films.Comment: 5 pages, 5 figure
Multistep Bloch-line-mediated Walker breakdown in ferromagnetic strips
A well-known feature of magnetic field driven dynamics of domain walls in
ferromagnets is the existence of a threshold driving force at which the
internal magnetization of the domain wall starts to precess -- a phenomenon
known as the Walker breakdown -- resulting in an abrupt drop of the domain wall
propagation velocity. Here, we report on micromagnetic simulations of magnetic
field driven domain wall dynamics in thin ferromagnetic strips with
perpendicular magnetic anisotropy which demonstrate that in wide enough strips
Walker breakdown is a multistep process: It consists of several distinct
velocity drops separated by short linear parts of the velocity vs field curve.
These features originate from the repeated nucleation, propagation and
annihilation of an increasing number of Bloch lines within the domain wall as
the driving field magnitude is increased. This mechanism arises due to
magnetostatic effects breaking the symmetry between the two ends of the domain
wall.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
Dynamic hysteresis in cyclic deformation of crystalline solids
The hysteresis or internal friction in the deformation of crystalline solids
stressed cyclically is studied from the viewpoint of collective dislocation
dynamics. Stress-controlled simulations of a dislocation dynamics model at
various loading frequencies and amplitudes are performed to study the stress -
strain rate hysteresis. The hysteresis loop areas exhibit a maximum at a
characteristic frequency and a power law frequency dependence in the low
frequency limit, with the power law exponent exhibiting two regimes,
corresponding to the jammed and the yielding/moving phases of the system,
respectively. The first of these phases exhibits non-trivial critical-like
viscoelastic dynamics, crossing over to intermittent viscoplastic deformation
for higher stress amplitudes.Comment: 5 pages, 4 figures, to appear in Physical Review Letter
Mimicking complex dislocation dynamics by interaction networks
Two-dimensional discrete dislocation models exhibit complex dynamics in
relaxation and under external loading. This is manifested both in the
time-dependent velocities of individual dislocations and in the ensemble
response, the strain rate. Here we study how well this complexity may be
reproduced using so-called Interaction Networks, an Artificial Intelligence
method for learning the dynamics of complex interacting systems. We test how to
learn such networks using creep data, and show results on reproducing
individual and collective dislocation velocities. The quality of reproducing
the interaction kernel is discussed
Predicting elastic and plastic properties of small iron polycrystals by machine learning
Deformation of crystalline materials is an interesting example of complex
system behaviour. Small samples typically exhibit a stochastic-like, irregular
response to externally applied stresses, manifested as significant
sample-to-sample variation in their mechanical properties. In this work we
study the predictability of the sample-dependent shear moduli and yield
stresses of a large set of small cube-shaped iron polycrystals generated by
Voronoi tesselation, by combining molecular dynamics simulations and machine
learning. Training a convolutional neural network to infer the mapping between
the initial polycrystalline structure of the samples and features of the
ensuing stress-strain curves reveals that the shear modulus can be predicted
better than the yield stress. We discuss our results in the context of the
sensitivity of the system's response to small perturbations of its initial
state
Barkhausen noise from formation of 360 domain walls in disordered permalloy thin films
Barkhausen noise in disordered ferromagnets is typically understood to
originate primarily from jerky field-driven motion of domain walls. We study
the magnetization reversal process in disordered permalloy thin films using
micromagnetic simulations, and find that the magnetization reversal process
consists of the gradual formation of immobile 360 domain walls via a
sequence of localized magnetization rotation events. The density of
360 domain walls formed within the sample as well as the statistical
properties of the Barkhausen jumps are controlled by the disorder strength
The effect of disorder on transverse domain wall dynamics in magnetic nanostrips
We study the effect of disorder on the dynamics of a transverse domain wall
in ferromagnetic nanostrips, driven either by magnetic fields or spin-polarized
currents, by performing a large ensemble of GPU-accelerated micromagnetic
simulations. Disorder is modeled by including small, randomly distributed
non-magnetic voids in the system. Studying the domain wall velocity as a
function of the applied field and current density reveals fundamental
differences in the domain wall dynamics induced by these two modes of driving:
For the field-driven case, we identify two different domain wall pinning
mechanisms, operating below and above the Walker breakdown, respectively,
whereas for the current-driven case pinning is absent above the Walker
breakdown. Increasing the disorder strength induces a larger Walker breakdown
field and current, and leads to decreased and increased domain wall velocities
at the breakdown field and current, respectively. Furthermore, for adiabatic
spin transfer torque, the intrinsic pinning mechanism is found to be suppressed
by disorder. We explain these findings within the one-dimensional model in
terms of an effective damping parameter increasing with the disorder
strength.Comment: 5 pages, 3 figure
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