The phase diagram and critical behaviour of the easy-plane antiferromagnet

Abstract

Neutron diffraction measurements have been performed to determine the magnetic phase diagram of the stacked-triangular antiferromagnet between T = 1.8 K and T = 6 K in static magnetic fields up to B = 5 T applied perpendicular to the c-axis. The substitution of a few per cent of for leads to a compensation of the weak Ising anisotropy of the pure compound, and induces an XY-anisotropy in the doped system. The magnetic phase diagram displays three different magnetically ordered phases with a multicritical point at B = 2 T and T = 4.3 K. The phase diagram and the spin configurations of the different phases are found to be consistent with theoretical predictions for stacked-triangular antiferromagnets with a weak easy-plane anisotropy. The spin ordering at zero field exhibits a structure in the basal plane with inherent XY- and chiral degeneracy. We have determined the critical exponent of the sublattice magnetization to be , which is close to the theoretical value of an XY chiral antiferromagnet.