  We report on {23}Na NMR studies of a honeycomb lattice antiferromagnet
Na2Ni2TeO6 by {23}Na nuclear spin-echo techniques. The {23}Na nuclear
spin-lattice relaxation rate 1/{23}T_1 exhibits critical divergence near a Neel
temperature T_N = 26 K, a narrow critical region, and a critical exponent w =
0.34 in 1/{23}T_1 = a (T/T_N - 1)^{-w} for Na2Ni2TeO6, and T_N = 18 K for
Na2(Ni{0.5}Cu{0.5})2TeO6. Although the uniform magnetic susceptibility of
Na2Ni2TeO6 exhibits a broad maximum at 35 K characteristic of low dimensional
spin systems, the NMR results indicate three dimensional critical phenomenon
around the Neel temperature.
