Dynamic Phase transitions in ferromagnetic systems : Some new phenomena

The thermodynamical behaviours of ferromagnetic systems in equilibrium are well studied. However, the ferromagnetic system, far from equilibrium, became an interesting field of research in last few decades. The ferromagnetic systems in the presence of a steady magnetic field are also studied by using standard tools of equilibrium statistical physics. The ferromagnet in the presence of time dependent magnetic field, shows various interesting phenomena, explored very recently. An usual response of a ferromagnet in presence of a sinusoidally oscillating magnetic field is the hysteresis. Apart from this hysteretic response, the nonequi-librium dynamic phase transition is a very interesting phenomenon. In this chapter, the nonequilibrium dynamic phase transitions, in model ferromagnetic systems in presence of time dependent magnetic field, are discussed. For this kind of nonequilibrium phase transition one cannot employ the standard techniques of equilibrium statistical mechanics. The recent developments in this direction are mainly based on numerical simulation (Monte Carlo). The Monte Carlo simulation, of kinetic Ising model in presence of sinusoidally oscillating (in time but uniform over space) magnetic field, is extensively performed to study the nonequilib-rium dynamic phase transition. The temperature variations of dynamic order parameter, dynamic specific heat, dynamic relaxation time etc. near the transition point are discussed. The appearance of a dynamic length scale and a dynamic time scale and their behaviours near the transition point are also discussed. All these studies indicate that this proposed dynamic transition is a nonequilibrium thermodynamic phase transition. The disorder (quenched) induced zero temperature (athermal) dynamic transition is studied in random field Ising ferromagnet. The dynamic transition in the Heisenberg ferromagnet is also studied. The nature of this transition in Heisenberg ferromagnet depends on the anisotropy and the polarisation of the applied time varying magnetic field. The anisotropic Heisenberg ferromagnet in presence of elliptically po-larised magnetic field shows multiple dynamic transitions. This multiple dynamic transitions in anisotropic Heisenberg ferromagnet are discussed here. Recent experimental evidences of dynamic transitions are also discussed very briefly.