Soft modes in singlet ground state ferromagnets

Abstract

The behaviour of magnetic excitons as a function of temperature is studied in cubic systems which have a ferromagnetic phase transition due to the exchange coupling between a singlet ground state and a triply degenerate excited state. The nature of the soft mode is discussed and found to be a zero-frequency electronic mode. It is shown that the singlet-triplet excitons do not exhibit full soft mode behaviour at T_c, and that the theory accounts qualitatively for the measurements of Birgeneau et al (1971) on Pr₃Tl and FCC Pr.