Phase Transitions in the Uniaxial Antiferromagnet

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.

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