Cu-NMR study of the quasi-one-dimensional magnet Cu3Mo2O9

A quasi-one-dimensional spin system Cu3Mo2O9, which has two spin degrees of freedom, the antiferromagnetic linear chain site and dimer-like site weakly interacting with each other, shows a 3D-antiferromagnetic order at T-N=7.9 K. We report the result of Cu-NMR on the single crystal of this system both in antiferromagnetic and paramagnetic states. In field-swept spectra, we observe two types of signals, which come from the chain site and the dimer-like site, respectively. With increasing resonant frequency, the signal from chain site shifts to lower magnetic field, while that from dimer-like site to higher field. This anomalous behaviour of the chain-site signal is explained in terms of the cancellation between the hyperfine field of about 8.5 T at the chain-site Cu and the applied field. A weak but finite hyperfine field of the dimer-like site indicates that there exists a localized spin on the site, indicating that it is not a singlet state.