Nonequilibrium Phase Transition in Ferromagnetic Model Systems

Abstract

In this thesis, I have mainly discussed the dynamical responses and behaviours of a ferromagnetic system under different kinds of magnetic field variations that are varied in time as well as in space, especially in the form of propagating magnetic waves and standing magnetic wave using Monte-Carlo simulation. These magnetic field waves cause the ferromagnet to undergo nonequilibrium phase transition; depending on the temperature, amplitude and wavelength of the magnetic field and the strength of anisotropy. In an Ising ferromagnet driven by standing magnetic field wave the high temperature phase is found to be quite different from that observed when propagating magnetic wave was passing through the Ising ferromagnet. Dynamical phases having similar attributes are formed in Blume-Capel (S = 1) ferromagnet when placed under these kinds of waves. However, in BC ferromagnet the strength of anisotropy affects the transition temperature as well as the morphological structure of the ferromagnetic spins. The studies regarding general characteristics of spin-S Ising ferromagnet excited by magnetic field waves revealed that the nature of dynamic phase transition is similar to the earlier studies but the transition temperature is found to decrease towards a limiting value with increase in the value of S. The values of critical exponents of spin - 1/2 Ising ferromagnet driven by propagating magnetic field waves are found to be very close to those obtained in Onsager’s solution for equilibrium ferro-para phase transition. This indicates that the nonequilibrium phase transition in an Ising ferromagnet driven by propagating magnetic wave belongs to the same Universality Class of equilibrium Ising ferromagnet.