Magnetic-field dependence of the phosphorus nuclear spin-relaxation rate in the quasi-two-dimensional XY antiferromagnet BaNi2(PO4)2

The phosphorus nuclear spin-lattice relaxation rate 1/T1 has been measured in the quasi-two-dimensional XY antiferromagnet BaNi2(PO4)2. The divergence observed just above the ordering temperature TN ≂ 23.6 K is analyzed within the context of the Kosterlitz–Thouless transition. For a magnetic field H perpendicular to the XY plane, the data presented here agree well with a recent model of freely moving vortices. The field dependence observed for H parallel suggests a drastic slowing down of the vortex velocity in that case.