Theory of Magnetic Phase Transitions

Abstract

Abstract The existence of magnetic order (collective magnetism) appearing in materials below a particular ordering temperature (e.g., the Curie temperature, T C , or the Neel temperature, T N ) points to a class of physical phenomena which are described as magnetic phase transition s. The thermodynamics of these phase transitions can be described by energy functions (expressed in terms of intensive or extensive variables), in terms of magnetization − temperature ( M − T ) phase diagrams, or in terms of critical exponents that describe the variation of thermodynamic properties (as a function of the order parameter) as the ordering temperature is approached. In this article, the thermodynamics of magnetic ordering transitions, the order of the transitions, critical exponents, and thermodynamic variables are described. Magnetic phase transitions within the framework of the mean field or Landau theory of phase transitions with discussion of several magnetic equations of state are presented.