Coupled order parameters, lattice disorder, and magnetic phase transitions

Abstract

A mean-field theory is developed which describes the effects of lattice disorder on magnetic phase transitions. Depending on the parameter values, three types of renormalization effects on $T_C$ can occur, i.e., weak, strong, or catastrophic; the last implies a first-order phase transition in both magnetic and lattice order parameter. Phase diagrams are obtained which show both the existence of critical triple points and the effects of phonon entropy in determining $T_C$. It is also shown that even in the weak-coupling limit, the lattice disordering process can be driven by magnetic entropy alone. The model is applied to MnBi, which undergoes a first-order magnetic phase transition well below its mean-field-value Curie temperature.