Towards an understanding of dynamic phase transitions

Abstract

We review the equivalencies and dissimilarities between ferromagnetic systems in thermodynamic equilibrium and those subjected to an external oscillating field. While the former undergo a thermodynamic phase transition at the Curie temperature, the latter show a dynamic phase transition at a critical period of the oscillating field. Over two decades of research have led to the conclusion that dynamic and thermodynamic phase transitions correspond to the same universality class and have analogous phase diagrams, among other similarities. Recently, the amplitude of the oscillating field driving the dynamic phase transition has emerged as a key variable, not only because it determines the value of the critical period directly and thus the speed of the critical dynamics, but also because it affects very substantially the degree of similarity in between thermodynamic and dynamic phase transitions. In particular, we discuss how low amplitudes of the oscillating field and corresponding slow critical dynamics lead to a suppression of the dynamic phase transition at surfaces and deviations from a Curie-Weiss type law at large periods. We also discuss the presence of metamagnetic anomalies in the paramagnetic dynamic phase and a severe shrinking of the critical region, which we have observed in magneto-optical Kerr effect experiments on in-plane uniaxial Co films and which we have analyzed in detail by means of mean-field calculations. At higher amplitudes of the oscillating field, and thus for faster critical dynamics, these phenomena are far less prominent and the equivalency picture between dynamic and thermodynamic phase transitions can be recovered.