Characterization of the spin gap nature in Na3Cu2SbO6 using 23Na NMR

► NMR results provide microscopic evidence for the existence of a spin gap in Na3Cu2SbO6. ► The NMR shift data can be fitted well to the AF–F alternating chain model. ► Analysis of the NMR spin-lattice relaxation rate yields a spin gap of 105K. We report the results of a Na23 nuclear magnetic resonance (NMR) study on Na3Cu2SbO6 at temperatures between 4 and 300K. 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 Tmax≃95K. 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 62K which is too small as compared to the value obtained from the neutron scattering measurement. On the other hand, a spin gap of about 112K 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 105K, in good agreement with the 8.9meV 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 Na3Cu2SbO6.