EQUATION OF STATE OF A FERROMAGNET IN CRITICAL REGION

It has been suggested that the equation of state of a ferromagnet in the critical region can be put in the form βmH = 1-tΔFM -11-tΔ-γt = Tc/T. F(v) is a function which characterizes the critical behavior, and γ and Δ are two parameters from which all the indices which describe thermodynamic behavior in the critical region can be determined. Griffiths and Helfand have considered possible froms of the function F(v). The present investigation makes use of the higher-order derivatives of magnetization with respect to field as T→T c+ to provide a series expansion for F(v) in powers of v(t<1); and of corresponding derivatives as T→Tc- to provide information about F(v) near the value corresponding to the spontaneous magnetization (t>1). The forms of F(v) for a number of lattices and models are compared. © 1968 The American Institute of Physics.