95 research outputs found
Nonequilibrium Phase Transition in the Kinetic Ising model: Absence of tricritical behaviour in presence of impurities
The nonequilibrium dynamic phase transition, in the two dimensional site
diluted kinetic Ising model in presence of an oscillating magnetic field, has
been studied by Monte Carlo simulation. The projections of dynamical phase
boundary (surface) are drawn in the planes formed by the dilution and field
amplitude and the plane formed by temperature and field amplitude. The
tricritical behaviour is found to be absent in this case.Comment: This paper is withdrawn due to some error
Form invariant Sommerfeld electrical conductivity in generalised d dimensions
The Sommerfeld electrical conductivity is calculated in d dimensions
following Boltzmann kinetic approach. At T=0, the mathematical form of the
electrical conductivity is found to remain invariant in any generalised spatial
(d) dimensions.Comment: To appear in Comm. Theo. Phys. (2011
Model and Statistical Analysis of the Motion of a Tired Random Walker in Continuum
The model of a tired random walker, whose jump-length decays exponentially in
time, is proposed and the motion of such a tired random walker is studied
systematically in one, two and three dimensional contin- uum. In all cases, the
diffusive nature of walker, breaks down due to tiring which is quite obvious.
In one dimension, the distribution of the displace- ment of a tired walker
remains Gaussian (as observed in normal walker) with reduced width. In two and
three dimensions, the probability distribution of displacement becomes
nonmonotonic and unimodal. The most probable displacement and the deviation
reduces as the tiring factor increases. The probability of return of a tired
walker, decreases as the tiring factor increases in one and two dimensions.
However, in three dimensions, it is found that the probability of return almost
insensitive to the tiring factor. The prob- ability distributions of first
return time of a tired random walker do not show the scale invariance as
observed for a normal walker in continuum. The exponents, of such power law
distributions of first return time, in all three dimensions are estimated for
normal walker. The exit probability and the probability distribution of first
passage time are found in all three dimensions. A few results are compared with
available analytical calculations for normal walker.Comment: 10 pages latex and 20 Latex figure
Polarised Electromagnetic wave propagation through the ferromagnet: Phase boundary of dynamic phase transition
The dynamical responses of ferromagnet to the propagating electromagnetic
field wave passing through it are modelled and studied here by Monte Carlo
simulation in two dimensional Ising ferromagnet. Here, the electromagnetic wave
is linearly polarised in such a way that the direction of magnetic field is
parallel to that of the magnetic momemts (spins). The coherent propagating mode
of spin-clusters is observed. The time average magnetisation over the full
cycle (time) of the field defines the order parameter of the dynamic
transition. Depending on the value of the temperature and the amplitude of the
propagating magnetic field wave, a dynamical phase transition is observed. The
dynamic transition was detected by studying the temperature dependences of the
dynamic order parameter, the variance of the dynamic order parameter, the
derivative of the dynamic order parameter and the dynamic specific heat. The
phase boundaries of the dynamic transitions were drawn for two different values
of the wave lengths of the propagating magnetic field wave. The phase boundary
was observed to shrink (inward) for lower speed of propagation of the EM wave.
The divergence of the releavant length scale was observed at the transition
point.Comment: 15 pages Latex, 6 figure
Comparison of meanfield and Monte Carlo approaches to dynamic hysteresis in Ising ferromagnets
The dynamical hysteresis is studied in the kinetic Ising model in the
presence of a sinusoidal magnetic field both by Monte Carlo simulation and by
solving the dynamical meanfield equation for the averaged magnetisation. The
frequency variations of the dynamic coercive field are studied below the
critical temperature. In both the cases, it shows a power law frequency
variation however it becomes frequency independent in the low frequency regime
for the meanfield case.Comment: 7 pages Latex (including figs.
Nonequilibrium Magnetisation Reversal by Periodic Impulsive Field in Ising Meanfield Dynamics
We studied the nonequilibrium magnetisation reversal in kinetic Ising
ferromagnets driven by periodic impulsive magnetic field in meanfield
approximation. The meanfield differential equation was solved by sixth order
Runge-Kutta-Felberg method. The periodicity and strength of the applied
impulsive magnetic field play the key role in magnetisation reversal. We
studied the minimum strength of impulsive field required for magnetisation
reversal at any temperature as a function of the periodicity of the impulsive
field. In the high temperature and small period this is observed to be linear.
The results are compared with that obtained from Monte Carlo simulation of a
three dimensional Ising ferromagnet.Comment: To appear in Physica Scripta (2010
Nonequilibrium phase transition in the kinetic Ising model driven by propagating magnetic field wave
The two dimensional ferromagnetic Ising model in the presence of a
propagating magnetic field wave (with well defined frequency and wavelength) is
studied by Mone Carlo simulation. This study differs from all of the earlier
studies done so far, where the oscillating magnetic field was considered to be
uniform in space. The time average magnetisation over a full cycle (the time
period) of the propagating magnetic field acts as the dynamic order parameter.
The dynamical phase transition is observed. The temperature variation of the
dynamic order parameter, the mean square deviation of the dynamic order
parameter, the dynamic specific heat and the derivative of the dynamic order
parameter are studied. The mean square deviation of the dynamic order
parameter, dynamic specific heat show sharp maxima near the transition point.
The derivative of dynamic order parameter shows sharp minimum near the
transition point. The transition temperature is found to depend also on the
speed of propagation of the magnetic field wave.Comment: To appear in Physica Scripta (2011
Standing spin wave mode in RFIM at T=0: Patterns and nonequilibrium phases
The dynamical responses of random field Ising model at zero temperature,
driven by standing magnetic field wave, is studied by Monte Carlo simulation in
two dimensions. The three different kinds of distribution of quenched random
field are used here, uniform, bimodal and Gaussian. In all cases, three
distinct dynamical phases were observed, namely, the pinned, structured and
random. In the pinned phase no spin flip is observed. In the structured phase
standing spin wave modes are observed. The random phase is shown no regular
pattern. For a fixed value of the amplitude of the standing magnetic field
wave, in the region of small quenched field, the system remains in a pinned
phase. In the intermediate range of values of random field, a standing spin
wave mode (structured phase) is observed. The regular pattern of this spin wave
mode disappears for higher values of random field yielding a random phase. The
comprehensive phase baundaries are drawn in all three cases. The boundary of
pinned phase are analytically calculated for uniform and bimodal types of
quenched random fields.Comment: 14 pages Late
Monte Carlo study of dynamic phase transition in Ising metamagnet driven by oscillating magnetic field
The dynamical responses of Ising metamagnet (layered antiferromagnet) in the
presence of a sinusoidally oscillating magnetic field are studied by Monte
Carlo simulation. The time average staggered magnetisation plays the role of
dynamic order parameter. A dynamical phase transition was observed and a phase
diagram was plotted in the plane formed by field amplitude and temperature. The
dynamical phase boundary is observed to shrink inward as the relative
antiferromagnetic strength decreases. The results are compared with that
obtained from pure ferromagnetic system. The shape of dynamic phase boundary
observed to be qualitatively similar to that obtained from previous meanfield
calculations.Comment: 12 pages latex, PC-12-2010, To appear in J. Mag. Mag. Mat. (2011
Random field Ising model swept by propagating magnetic field wave: Athermal nonequilibrium phase diagram
The dynamical steady state behaviour of the random field
Ising ferromagnet swept by a propagating magnetic field wave is studied at
zero temperature by Monte Carlo simulation in two dimensions. The distribution
of the random field is bimodal type. For a fixed set of values of the frequency
and wavelength of propagating magnetic field wave and the strength of the
random field, four distinct dynamical steady states or nonequilibrium phases
were identified. These four nonequilibrium phases are characterised by
different values of structure factors. State of first kind, where all spins are
parallel (up). The second one is, the propagating type, where the sharp strips
formed by parallel spins are found to move coherently. The third one is also
propagating type, where the boundary of the strips of spins is not very sharp.
The fourth kind, shows no propagation of stripes of magnetic spins, forming a
homogeneous distribution of up and down spins. This is disordered phase The
appearance of these four dynamical phases or modes depends on the value of the
amplitude of propagating magnetic field wave and the strength of random
(static) field. A phase diagram has also been drawn, in the plane formed by the
amplitude of propagating field and the strength of random field. It is checked
that, the existence of these dynamical phases is neither a finite size effect
nor a transient phenomenon.Comment: 13 pages Latex, To Appear in J. Magn. Magn. Mater. (2013
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