Spin dynamics in a triangular antiferromagnet UNi₄B

11B NMR and magnetic susceptibility measurements have been performed in order to investigate the spin dynamics of a triangular-lattice XY antiferromagnet UNi4B. In this compound, the partially ordered state appears below TN = 20 K with a peculiar vortex-like structure. Magnetic susceptibility shows that TN is slightly lower than the round maximum of the magnetic susceptibility suggesting the two dimensional character of this compound. The temperature dependence of the relaxation rate T-1 1 above TN is well reproduced by the theoretical model that were discussed in the Kosterlitz-Thouless- Berezinskii transition. Thus the temperature dependence of T-1 1 suggests that the spin dynamics in UNi₄B above TN is dominated by the motion of the unbound vortices.