NMR study of pyrochlore lattice antiferromagnet, melanothallite Cu ₂OCl₂

The melanothallite Cu₂OCl₂ is a new example of pyrochlore-like antiferromagnet, which is composed of 3d transition metal electrons. We performed Cu- and Cl-NMR experiments on powder samples of Cu ₂OCl₂ below transition temperature T_N = 70 K and we observed six resonant peaks of Cu nuclei, which are composed of three symmetric peaks corresponding to ⁶³Cu and three corresponding to ⁶⁵Cu. The Cu nuclei feel the strong hyperfine fields because of ordered magnetic moments and the electric field gradients. We determined the spin structure by analyzing the Cu-NMR spectra. The melanothallite has an all-in-all-out spin structure. The spin lattice relaxation rates T ₁^-1 of Cu- and Cl-NMR in the ordered phase are proportional to the temperature; This suggests that although long-range ordering occurs at rather high temperature, the large spin fluctuations caused by the geometrical frustration still remain.