23Na Nuclear Spin-Lattice Relaxation Studies of Na2Ni2TeO6

We report on Na-23 NMR studies of the honeycomb lattice antiferromagnet Na2Ni2TeO6 by Na-23 nuclear spin-echo techniques. The Na-23 nuclear spin-lattice relaxation rate 1/T-23(1) exhibits critical divergence near the Neel temperature T-N = 26 K, a narrow critical region, and the critical exponent w = 0.34 in 1/T-23(1) alpha (T/TN - 1)(-w) for Na2Ni2TeO6, and T-N = 18 K for Na-2(Ni0.5Cu0.5)(2)TeO6. Although the uniform magnetic susceptibility of Na2Ni2TeO6 exhibits a broad maximum at 35 K, which is the characteristic of low-dimensional spin systems, the NMR results indicate a three-dimensional criticalphenomenon near the Neel temperature.