NMR evidence for the spin gap behavior in Cu₂PO₄OH

We report on the results of a P 31 nuclear-magnetic-resonance (NMR) study on Cu₂PO₄OH 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 ≃ 110 K. Below T_max, the NMR shifts and spin-lattice relaxation rates clearly indicate activated responses, confirming the existence of a spin gap in Cu ₂PO₄OH. The experimental NMR data can be well fitted to the square spin tetramer (SQST) model, yielding a spin gap size of about 140 K. It thus points out that the nearly isolated SQST picture is proper for the understanding of the spin gap nature in Cu₂PO₄OH.