Neutron scattering study of the magnetic excitations in FeBr2

Abstract
The magnetic excitations in FeBr2 have been studied between 2K and 9K using inelastic neutron scattering. The spin waves may be described by a simple S=1 Hamiltonian with an anisotropy field g mu BHA=0.428+or-0.016 THz, intra-plane isotropic interactions of 2J1=0.152+or-0.008 THz, 2J2=-0.051+or-0.006 THz and an inter-planar interaction 2J'1=-0.06+or-0.005 THz. The spin-wave spectrum is quite similar to FeCl2, although slightly less two-dimensional. At T>2K a second type of excitation is observed, which interacts strongly with the spin waves and is thought to correspond to the single ion transition from the Sz=0 to Sz=+1 state of the ground-state triplet.