One- and two-magnon excitations in FeBr₂

The magnetic excitations in the typical S=1 metamagnet FeBr₂ are studied by Raman scattering spectroscopy for temperatures between 6K and 25K and the spectra are interpreted using an interacting magnon theory. A detailed comparison between theory and experiment for the frequency and damping of both acoustic and optic one-magnon branches shows good agreement for T<or approximately=0.8 T_N. The measured optic magnon integrated intensity is found to exceed the acoustic magnon intensity at higher temperatures. This result is discussed using a linear magnon theory, which gives qualitative agreement with experiment and indicates that quadratic magneto-optic coupling plays an important role. Three other lines observed at higher frequencies are interpreted in terms of continuum and bound two-magnon states. The existence of bound states in FeBr₂ is attributed to its large single-ion anisotropy.