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

Abstract

Magnetic order and low-energy spin dynamics in the zero field ground state of Ni${}_{3}$V${}_{2}$O${}_{8}$ are revealed in elastic and inelastic neutron scattering experiments. Neutron diffraction shows that below $T=2.3$ K the Ni${}^{2+}$ moments (spin $S=1$) order in a cycloid pattern with incommensurate wave vector ${\mathbit{k}}_{\text{ICM}}=(0,1,\ensuremath{\tau})$, where $\ensuremath{\tau}=0.4030\ifmmode\pm\else\textpm\fi{}0.0004$, which is superimposed on a commensurate antiferromagnetic spin arrangement with ${\mathbit{k}}_{\text{CM}}=(0,0,0)$. Three spin wave modes are discerned below $E\ensuremath{\sim}3$ 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.