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 T_N = 20 K with a peculiar vortex-like structure. Magnetic susceptibility shows that T_N 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^-1₁ above T_N is well reproduced by the theoretical model that were discussed in the Kosterlitz-Thouless-Berezinskii transition. Thus the temperature dependence of T^-1₁ suggests that the spin dynamics in UNi₄B above T_N is dominated by the motion of the unbound vortices.