1H-NMR Study of Spin-1/2 Triple-chain Magnet Cu3(OH)4MoO4

We have studied a natural mineral Cu3(OH)4MoO4 (mineral name: szenicsite), which is characterized as S = 1/2 triple-chain antiferromagnet, by measurements of the magnetic susceptibility χ and 1H-NMR in order to determine the low temperature magnetic state from a microscopic viewpoint. χ(T) was fitted fairly well by the sum of isolated uniform antiferromagnetic chains and a small amount of paramagnetic impurities. The width of 1H-NMR spectrum increased with decreasing temperature, suggesting inhomogeneous broadening due to impurity moments. The shift of the spectrum showed a maximum at approximately 70 K and continuously decreased with decreasing temperature to the lowest temperature of 1.5 K. 1D nature of this compound was also suggested from temperature and field dependences of the nuclear spin-lattice relaxation time T1. We also found that the nuclear spin-spin relaxation time T2 depended on the RF pulse power, which suggests that a relaxation process among proton spins contributes significantly to the T2. No magnetic long-range ordering was found. These results suggest that the intrinsic magnetic properties of Cu3(OH)4MoO4 in the temperature range down to 1.5 K are close to those of uniform antiferromagnetic chains without a spin gap.