Characterization of the spin gap nature in Na ₃Cu ₂SbO ₆ using ²³Na NMR

We report the results of a Na23 nuclear magnetic resonance (NMR) study on Na ₃Cu ₂SbO ₆ at temperatures between 4 and 300 K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a broad maximum at T _max 95 K. While the NMR shift data can be fitted well to both antiferromagnetic- antiferromagnetic (AF-AF) and antiferromagnetic-ferromagnetic (AF-F) alternating chain models, the former yields a spin gap of 62 K which is too small as compared to the value obtained from the neutron scattering measurement. On the other hand, a spin gap of about 112 K based on the AF-F alternating chain scenario is quite consistent with the reported value. In addition, the employment of the AF-F alternating chain picture to analyze the spin-lattice relaxation rate reveals a gap size of 105 K, in good agreement with the 8.9 meV peak observed in the inelastic neutron scattering experiment. With this accordance, we point out that the AF-F alternating chain picture is more appropriate for the understanding of the spin gap nature in Na ₃Cu ₂SbO ₆.