Determining The Berezinskii-Kosterlitz-Thouless Coherence Length In Bani2v2o8 By V-51 Nmr

The magnetic properties of BaNi2V2O8 are studied by electron paramagnetic resonance (EPR) and V-51 nuclear magnetic resonance (NMR). The divergence of the T-51(1) rate on approaching TN in the paramagnetic phase displays the exponential correlation length expected for the Berezinskii-Kosterlitz-Thouless transition. The critical power-law behavior can be restored by applying a strong magnetic field perpendicular to the hexagonal axis, destroying the XY anisotropy. The comparison of the EPR and NMR measurements at several magnetic fields yields a consistent description in terms of the Berezinskii-Kosterlitz-Thouless scenario in the temperature range above the antiferromagnetic phase transition of BaNi2V2O8.