The field induced magnetization reorientation in weak ferromagnets—III: The case of non-equivalent single ion anisotropy

The phase transitions in a single-ion anisotropy type weak ferromagnet at 0°K are studied using a two sublattice model and molecular field theory. In an external field applied along the antiferromagnetic axis, there exist three stable phases, denoted as weak ferromagnetic (WF), spin-flop (SF), and metamagnetic (MM). The character of the WF to SF transition changes from first to second order as the angle between a sublattice uniaxial anisotropy axis and the antiferromagnetic axis is increased. The tricritical field for this phase transition is proportional to the one-half power of the uniaxial anisotropy field, when the latter is much smaller than the exchange field. The transitions to and from the MM phase are always of first order. The former can result from either a WF or a SF phase instability threshold being reached. The latter always results in a transition to the WF phase. All three phases can, under specified conditions, coexist for a range of applied field values. In this case, an instability of the WF phase always results in a first order transition to the SF phase.