Quantum anomalies in the phase diagrams of easy-plane antiferromagnets in a magnetic field

We obtain the phase boundaries for easy-plane antiferromagnets with a single-ion anisotropy in a longitudinal magnetic field under a molecular field approximation for arbitrary values of the spin and the ratio of the anisotropy to the exchange. It is shown that for a finite spin S the phase diagram exhibits anomalies which disappear in the classical limit S → ∞.

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