High-field NMR study of the spin correlations in the spin-cluster mineral Na2Cu3O(SO4)3

We report a nuclear magnetic resonance study of the spin correlations in the spin-cluster-based mineral Na2Cu3O(SO4)3 with magnetic fields ranging from 1 to 33 T. The long-range magnetic order is observed from both the sudden spectral broadening at TN and the critical slowing down behavior in the temperature dependence of spin-lattice relaxation rates [1/T1(T )]. The hump behavior of 1/T1(T ) persists to & mu;0H = 7.25 T, above which a spin excitation gap is observed from the thermally activated temperature dependence of 1/T1. The gap size shows a linear field dependence, whose slope and intercept respectively yield an effective magnetic moment of 2.54 & mu;B and a 0.94-meV spin excitation gap under zero magnetic field. These results indicate the existence of short-range order and prominent easy-plane spin anisotropy, which are important for understanding the spin excitation spectrum in A2Cu3O(SO4)3.