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 ${\mathrm{Na}}_{2}{\mathrm{Cu}}_{3}\mathrm{O}{({\mathrm{SO}}_{4})}_{3}$ with magnetic fields ranging from 1 to 33 T. The long-range magnetic order is observed from both the sudden spectral broadening at ${T}_{N}$ and the critical slowing down behavior in the temperature dependence of spin-lattice relaxation rates $[1/{T}_{1}(T)]$. The hump behavior of $1/{T}_{1}(T)$ persists to ${\ensuremath{\mu}}_{0}H=7.25$ T, above which a spin excitation gap is observed from the thermally activated temperature dependence of $1/{T}_{1}$. The gap size shows a linear field dependence, whose slope and intercept respectively yield an effective magnetic moment of 2.54 ${\ensuremath{\mu}}_{B}$ 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 ${A}_{2}{\mathrm{Cu}}_{3}\mathrm{O}{({\mathrm{SO}}_{4})}_{3}$.