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 k_ICM=(0,1,τ), where τ=0.4030±0.0004, which is superimposed on a commensurate antiferromagnetic spin arrangement with k_CM=(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.