Driven spin wave modes in XY ferromagnet: non-equilibrium phase transition

Abstract

The dynamical responses of XY ferromagnet driven by linearly polarised propagating and standing magnetic field wave have been studied by Monte Carlo simulation in three dimensions. In the case of propagating magnetic field wave (with specified amplitude, frequency and the wavelength), the low temperature dynamical mode is a propagating spin wave and the system becomes structureless (or random) in the high temperature. A dynamical symmetry breaking phase transition is observed at a finite (nonzero) temperature. This symmetry breaking is confirmed by studying the statistical distribution of the angle of the spin vector. The dynamic nonequilibrium transition temperature was found to decrease as the amplitude of the propagating magnetic field wave increased. A comprehensive phase boundary is drawn in the plane formed by temperature and amplitude of propagating field wave. The phase boundary was observed to shrink (in the low temperature side) for longer wavelength of the propagating magnetic wave. In the case of standing magnetic field wave, the low temperature excitation is a standing spin wave which becomes structureless (or random) in the high temperature. Here also, like the case of propagating magnetic wave, a dynamical symmetry breaking nonequilibrium phase transition was observed. A comprehensive phase boundary was drawn. Unlike the case of propagating magnetic wave, the phase boundary does not show any systematic variation with the wavelength of the standing magnetic field wave. In the limit of vanishingly small amplitude of the field, the phase boundaries approach the recent Monte Carlo estimate of equilibrium transition temperature. I. Introduction:The nonequilibrium responses of Ising ferromagnet to an oscillating (in time but uniform over the space) magnetic field is an interesting field of modern research [1,2]. The nonequilibrium phase transition is one major focus of the investigation. Some important studies may be reported below. The existence of the growth of correlation near the transition was reported [3]. The bulk and surface critical behaviours were studied recently[4] and those are found to belong to different universality class. The anomalous metamagnetic fluctuations near the transition were studied recently [5]. This study was supported by Monte Carlo simulation [6]. Experimentally, a notable transient behaviour was found [7], in the uniaxial cobalt film, for the time period of the field which is faster than a critical time. This is related to the the existence of first order transition. All these studies, mentioned above, are signatures of the current interest in the field of nonequilibrium responses of ferromagnets driven by time varying external magnetic field.One common and important feature of the above mentioned studies, is the time dependence of the external magnetic field, which keeps the system far away from the equilibrium. However, recently the interests have been taken in the case, where the driving magnetic field has both spatial and temporal variations. This spatio-temporal vari...