Random field Ising model swept by propagating magnetic field wave: Athermal nonequilibrium phasediagram

Abstract

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, wavelength and amplitude 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 or phase of first kind, where all spins are parallel (up). This phase is a frozen or pinned where the propagating field has no effect. 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 strips of magnetic spins, forming a homogeneous distribution of up and down spins. This is disordered phase. The existence 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 also checked that, the existence of these dynamical phases is neither a finite size effect nor a transient phenomenon. The behaviour of Ising ferromagnet with randomly quenched magnetic field is an active field of modern research. The zero temperature ferro-para transition [1] by the random field, made it more interesting. A simple model was proposed to understand the hysteresis incorporating the return point memory [2] and Barkhausen noise [3]. Later, the statistics [4] and the dynamic critical behaviour [5] of Barkhausen avalanches were studied in random field Ising model (RFIM). The hysteresis loop was exactly determined in one dimensional RFIM [6]. The RFIM was also studied [7] in the Bethe lattice.The dynamics of the domain wall in RFIM is another area of interest in modern research of statistical physics. The motion of domain wall shows very interesting depinning transition at zero temperature. Due to the energy barriers created by disorder (random field), the domain wall is pinned and the motion of the domain wall remains stopped upto a critical field [8,9]. However, at any finite temperature the depinning transition is softened and the thermal fluctuation assists to overcome the energy barrier. Thus with the application of sufficiently small amount of field, the motion of domain wall can reach a nonzero mean velocity, which is known as creep motion [10,11,12]. Very recently, the creep motion of domain wall in the two dimensional RFIM was studied (by Monte Carlo simulations) with a driving field[13] and observed field-velocity relationship and estimated the creep exponent.All the above mentioned studies are done with constant and uniform magnetic field ...

Spin flip statistics and spin wave interference patterns in Ising ferromagnetic films: A Monte Carlo study

“…The propagation of plane spin wave is observed [14] , [15] and dynamic phase transition is found [16] in Ising ferromagnetic film driven by plane propagating magnetic wave. Recently the propagation of spin clusters and existence of various dynamic phases are observed [17] in random field Ising ferromagnet. In normal ferromagnet, the propagation of circular spin waves are observed [18] in Ising ferromagnet and multiple nonequilibrium phase transitions are found.…”