Magnetic properties of uniaxial antiferromagnets

Abstract

The magnetic phase diagrams of pure and diluted uniaxial Heisenberg antiferromagnets with crystal-field anisotropy of the form D(Sz)2 are considered. Treating the anisotropy exactly and only the exchange interaction in the mean-field approximation, it is shown that for S=1, the phase transition between the paramagnetic and the spin-flop phase is first order for values of the anisotropy parameter D>Dc and is second order only for D 3/2 no tricritical point is observed in the mean-field approximation. The antiferromagnetic to spin-flop phase stability limit never exceeds the antiferromagnetic to paramagnetic critical field.