Phase Transitions in the Uniaxial Antiferromagnet

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Abstract
We have studied the transitions induced by fields of arbitrary direction between the antiferromagnetic, the spin‐flop, and the paramagnetic phase of a two‐sublattice uniaxial antiferromagnet at zero temperature in molecular‐field approximation. We find that the phase boundary between the antiferromagnetic and the spin‐flop phase in the H plane extends only to a maximum angle with respect to the easy axis of order (anisotropy energy/exchange energy), where it ends in critical points. This behavior explains the sensitive angular dependence of ultrasonic absorption and magnetic resonance experiments.