One- and two-magnon excitations in antiferromagnet PbFeBO4

The linear spin-wave theory study of PbFeBO4 spin dynamics is presented. It is shown that the modes observed in Raman scattering experiments below Neel temperature are optical magnon and two-magnon excitations. Based on the magnon energy, two-magnon band lineshape, and Weiss temperature, the consistent set of the exchange coupling constants up the third neighbor is derived and compared with the results of ab initio calculations. The small deviation of the observed two-magnon band from the one-magnon density of states suggests a surprisingly negligible role of magnon-magnon interactions.