Collinear spin density wave order and anisotropic spin fluctuations in the frustrated J_1-J_2 chain magnet NaCuMoO_4(OH)

摘要

The phase diagram of the quasi-one-dimensional magnet NaCuMoO_4(OH) is established through single-crystal NMR and heat-capacity measurements. The ~(23)Na and ~1H NMR experiments indicate a spiral and a collinear spin density wave (SDW) order below and above B_c = 1.5-1.8 T, respectively. Moreover, in the paramagnetic state above the SDW transition temperature, the nuclear spin-lattice relaxation rate 1 / T_1 indicates anisotropic spin fluctuations that have gapped excitations in the transverse spectrum but gapless ones in the longitudinal spectrum. These static and dynamic properties are well described by a theoretical model assuming quasi-one-dimensional chains with competing ferromagnetic nearest-neighbor interactions J and antiferromagnefic next-nearest-neighbor interactions J_2 (J_1-J_2 chains). Because of the excellent crystal quality and good one-dimensionality, NaCuMoO_4(OH) is a promising compound to elucidate the unique physics of the frustrated J_1-J_2 chain.