Induced staggered magnetic fields in antiferromagnets

Abstract

Uniform magnetic fields may couple in higher order to the antiferromagnetic order parameter in a large class of compensated antiferromagnets. We describe this coupling as an induced staggered field. In this paper we give the general rules which determine when the coupling is allowed by the symmetry of the order parameter. We also discuss the dependence of the coupling on field direction. Some implications of these results for the behavior of transition-metal fluorides and for metamagnetic tricritical points are given. In particular we find that there is no induced staggered field in systems such as Fe${\mathrm{Cl}}_2$ while a coupling is expected in FeC${\mathrm{O}}_3$ and Fe${\mathrm{F}}_2$, as well as in the recently discussed case of dysprosium aluminum garnet. In all these systems, however, the staggered field should vanish along certain symmetry directions for which metamagnetic tricritical points should exist.