Collinear spin density wave order and anisotropic spin fluctuations in the frustrated J1−J2 chain magnet NaCuMoO4(OH).

The phase diagram of the quasi-one-dimensional magnet NaCuMoO4(OH) is established through single-crystal NMR and heat-capacity measurements. The 23Na and 1H NMR experiments indicate a spiral and a collinear spin density wave (SDW) order below and above Bc = 1.5–1.8 T, respectively. Moreover, in the paramagnetic state above the SDW transition temperature, the nuclear spin-lattice relaxation rate 1/T1 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 J1 and antiferromagnetic next-nearest-neighbor interactions J2 (J1−J2 chains). Because of the excellent crystal quality and good one-dimensionality, NaCuMoO4(OH) is a promising compound to elucidate the unique physics of the frustrated J1−J2 chain. [ABSTRACT FROM AUTHOR]