259 research outputs found
Mechanism for nonequilibrium symmetry breaking and pattern formation in magnetic films
Magnetic thin films exhibit a strong variation in properties depending on
their degree of disorder. Recent coherent x-ray speckle experiments on magnetic
films have measured the loss of correlation between configurations at opposite
fields and at the same field, upon repeated field cycling. We perform finite
temperature numerical simulations on these systems that provide a comprehensive
explanation for the experimental results. The simulations demonstrate, in
accordance with experiments, that the memory of configurations increases with
film disorder. We find that non-trivial microscopic differences exist between
the zero field spin configuration obtained by starting from a large positive
field and the zero field configuration starting at a large negative field. This
seemingly paradoxical beahvior is due to the nature of the vector spin dynamics
and is also seen in the experiments. For low disorder, there is an instability
which causes the spontaneous growth of line-like domains at a critical field,
also in accord with experiments. It is this unstable growth, which is highly
sensitive to thermal noise, that is responsible for the small correlation
between patterns under repeated cycling. The domain patterns, hysteresis loops,
and memory properties of our simulated systems match remarkably well with the
real experimental systems.Comment: 12 pages, 10 figures Added comparison of results with
cond-mat/0412461 and some more discussio
Effects of Domain Wall on Electronic Transport Properties in Mesoscopic Wire of Metallic Ferromagnets
We study the effect of the domain wall on electronic transport properties in
wire of ferromagnetic 3 transition metals based on the linear response
theory. We considered the exchange interaction between the conduction electron
and the magnetization, taking into account the scattering by impurities as
well. The effective electron-wall interaction is derived by use of a local
gauge transformation in the spin space. This interaction is treated
perturbatively to the second order. The conductivity contribution within the
classical (Boltzmann) transport theory turns out to be negligiblly small in
bulk magnets, due to a large thickness of the wall compared with the fermi
wavelength. It can be, however, significant in ballistic nanocontacts, as
indicated in recent experiments. We also discuss the quantum correction in
disordered case where the quantum coherence among electrons becomes important.
In such case of weak localization the wall can contribute to a decrease of
resistivity by causing dephasing. At lower temperature this effect grows and
can win over the classical contribution, in particular in wire of diameter
, being the inelastic diffusion
length. Conductance change of the quantum origin caused by the motion of the
wall is also discussed.Comment: 30 pages, 4 figures. Detailed paper of Phys. Rev. Lett. 78, 3773
(1997). Submitted to J. Phys. Soc. Jp
Spin Precession and Avalanches
In many magnetic materials, spin dynamics at short times are dominated by
precessional motion as damping is relatively small. In the limit of no damping
and no thermal noise, we show that for a large enough initial instability, an
avalanche can transition to an ergodic phase where the state is equivalent to
one at finite temperature, often above that for ferromagnetic ordering. This
dynamical nucleation phenomenon is analyzed theoretically. For small finite
damping the high temperature growth front becomes spread out over a large
region. The implications for real materials are discussed.Comment: 4 pages 2 figure
Deep Spin-Glass Hysteresis Area Collapse and Scaling in the Ising Model
We investigate the dissipative loss in the Ising spin glass in three
dimensions through the scaling of the hysteresis area, for a maximum magnetic
field that is equal to the saturation field. We perform a systematic analysis
for the whole range of the bond randomness as a function of the sweep rate, by
means of frustration-preserving hard-spin mean field theory. Data collapse
within the entirety of the spin-glass phase driven adiabatically (i.e.,
infinitely-slow field variation) is found, revealing a power-law scaling of the
hysteresis area as a function of the antiferromagnetic bond fraction and the
temperature. Two dynamic regimes separated by a threshold frequency
characterize the dependence on the sweep rate of the oscillating field. For
, the hysteresis area is equal to its value in the adiabatic
limit , while for it increases with the
frequency through another randomness-dependent power law.Comment: 6 pages, 6 figure
Ising Dynamics with Damping
We show for the Ising model that is possible construct a discrete time
stochastic model analogous to the Langevin equation that incorporates an
arbitrary amount of damping. It is shown to give the correct equilibrium
statistics and is then used to investigate nonequilibrium phenomena, in
particular, magnetic avalanches. The value of damping can greatly alter the
shape of hysteresis loops, and for small damping and high disorder, the
morphology of large avalanches can be drastically effected. Small damping also
alters the size distribution of avalanches at criticality.Comment: 8 pages, 8 figures, 2 colum
Barkhausen noise from zigzag domain walls
We investigate the Barkhausen noise in ferromagnetic thin films with zigzag
domain walls. We use a cellular automaton model that describes the motion of a
zigzag domain wall in an impure ferromagnetic quasi-two dimensional sample with
in-plane uniaxial magnetization at zero temperature, driven by an external
magnetic field. The main ingredients of this model are the dipolar spin-spin
interactions and the anisotropy energy. A power law behavior with a cutoff is
found for the probability distributions of size, duration and correlation
length of the Barkhausen avalanches, and the critical exponents are in
agreement with the available experiments. The link between the size and the
duration of the avalanches is analyzed too, and a power law behavior is found
for the average size of an avalanche as a function of its duration.Comment: 11 pages, 12 figure
Brownian forces in sheared granular matter
We present results from a series of experiments on a granular medium sheared
in a Couette geometry and show that their statistical properties can be
computed in a quantitative way from the assumption that the resultant from the
set of forces acting in the system performs a Brownian motion. The same
assumption has been utilised, with success, to describe other phenomena, such
as the Barkhausen effect in ferromagnets, and so the scheme suggests itself as
a more general description of a wider class of driven instabilities.Comment: 4 pages, 5 figures and 1 tabl
Subharmonics and Aperiodicity in Hysteresis Loops
We show that it is possible to have hysteretic behavior for magnets that does
not form simple closed loops in steady state, but must cycle multiple times
before returning to its initial state. We show this by studying the
zero-temperature dynamics of the 3d Edwards Anderson spin glass. The specific
multiple varies from system to system and is often quite large and increases
with system size. The last result suggests that the magnetization could be
aperiodic in the large system limit for some realizations of randomness. It
should be possible to observe this phenomena in low-temperature experiments.Comment: 4 pages, 3 figure
Thermodynamics as a nonequilibrium path integral
Thermodynamics is a well developed tool to study systems in equilibrium but
no such general framework is available for non-equilibrium processes. Only hope
for a quantitative description is to fall back upon the equilibrium language as
often done in biology. This gap is bridged by the work theorem. By using this
theorem we show that the Barkhausen-type non-equilibrium noise in a process,
repeated many times, can be combined to construct a special matrix
whose principal eigenvector provides the equilibrium distribution. For an
interacting system , and hence the equilibrium distribution, can be
obtained from the free case without any requirement of equilibrium.Comment: 15 pages, 5 eps files. Final version to appear in J Phys.
Magnetic hysteresis in Ising-like dipole-dipole model
Using zero temperature Monte Carlo simulations we have studied the magnetic
hysteresis in a three-dimensional Ising model with nearest neighbor exchange
and dipolar interaction. The average magnetization of spins located inside a
sphere on a cubic lattice is determined as a function of magnetic field varied
periodically. The simulations have justified the appearance of hysteresis and
allowed us to have a deeper insight into the series of metastable states
developed during this process.Comment: REVTEX, 10 pages including 4 figure
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