Ginzburg-Landau theory for the magnetic and structural transitions in La 1-y (Ca 1-x Sr x ) y MnO 3

Abstract

We present a phenomenological theory for the ferromagnetic transition temperature, the magnetic susceptibility at high temperatures, and the structural distortion in the La 1-y (Ca 1-x Sr x ) y MnO 3 system. We construct a GinzburgLandau free energy that describes the magnetic and the structural transitions, and a competition between them. The parameters of the magnetic part of the free energy are derived from a mean-field solution of the magnetic interaction for arbitrary angular momentum. The theory provides a qualitative description of the observed magnetic and structural phase transitions as functions of Sr-doping level (x) for y = 0.25.