Incommensurability and spin dynamics in the low-temperature phases of Ni3V2O8

Magnetic order and low-energy spin dynamics in the zero field ground state of Ni3V2O8 are revealed in elastic and inelastic neutron scattering experiments. Neutron diffraction shows that below T = 2.3 K the Ni2+ moments (spin S = 1) order in a cycloid pattern with incommensurate wave vector kICM = (0,1,τ ), where τ = 0.4030 ± 0.0004, which is superimposed on a commensurate antiferromagnetic spin arrangement with kCM = (0,0,0). Three spin wave modes are discerned below E ∼ 3 meV in inelastic measurements and qualitatively described by a model Hamiltonian that involves near neighbor exchange, local anisotropy, and a small biquadratic coupling between the spine and cross-tie sites. Results from both elastic and inelastic scattering experiments suggest that the two sublattices on spine and cross-tie sites are largely decoupled.