748 research outputs found
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
Continuum mesoscale theory inspired by plasticity
We present a simple mesoscale field theory inspired by rate-independent
plasticity that reflects the symmetry of the deformation process. We
parameterize the plastic deformation by a scalar field which evolves with
loading. The evolution equation for that field has the form of a
Hamilton-Jacobi equation which gives rise to cusp-singularity formation. These
cusps introduce irreversibilities analogous to those seen in plastic
deformation of real materials: we observe a yield stress, work hardening,
reversibility under unloading, and cell boundary formation.Comment: 7 pages, 5 .eps figures. submitted to Europhysics Letter
Work distributions in the T=0 Random Field Ising Model
We perform a numerical study of the three-dimensional Random Field
Ising Model at T=0. We compare work distributions along metastable
trajectories obtained with the single-spin flip dynamics with the distribution
of the internal energy change along equilibrium trajectories. The goal is to
investigate the possibility of extending the Crooks fluctuation theorem to zero
temperature when, instead of the standard ensemble statistics, one considers
the ensemble generated by the quenched disorder. We show that a simple
extension of Crooks fails close to the disordered induced equilibrium phase
transition due to the fact that work and internal energy distributions are very
asymmetric
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
A cluster model with random anisotropy for hysteresis jumps in CeNiCu alloys
Some Cerium compounds exhibit hysteresis cycles with sharp macroscopic jumps
in the magnetization at very low temperatures. This effect is attributed to the
formation of clusters in which the anisotropy competes with the applied
magnetic field. Here, we present a simple model where a lattice of
ferromagnetically coupled spins is separated in clusters of random sizes and
with random anisotropy. Within this model, we obtain hysteresis cycles
presenting jumps that behave in a similar way that the experimental ones, and
that disappear when increasing the temperature. The results are in good
agreement with the hysteresis cycles measured at very low temperatures in
CeNiCu and the comparison with these experimental results allows
to discriminate the relative importance of the mechanisms driving the thermal
evolution of the cycles.Comment: Accepted in PR
Analysis of wasp-waisted hysteresis loops in magnetic rocks
The random-field Ising model of hysteresis is generalized to dilute magnets
and solved on a Bethe lattice. Exact expressions for the major and minor
hysteresis loops are obtained. In the strongly dilute limit the model provides
a simple and useful understanding of the shapes of hysteresis loops in magnetic
rock samples.Comment: 11 pages, 4 figure
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