Induced moment antiferromagnet in an external magnetic field—II: Green's function theory

A two level two sublattice antiferromagnetic induced moment system is examined in the Green's function theory in the random phase approximation. Attentions are drawn to the effects of an externally applied magnetic field. It is shown that a “soft mode” behavior occurs at the transition field similar to that observed at the transition temperature in the absence of a field. The transition becomes first order in the RPA. The sublattice magnetizations and the transition field are significantly lowered compared to the molecular field theory results.