Temperature renormalization of the magnon energies of FeBr 2

Abstract

The temperature renormalization of the magnon energies is investigated in FeBr 2, a S = 1 Heisenberg ferromagnet with rather large single-ion and exchange anisotropies. Two theories are applied: a self-consistently-renormalized (SCR) spin-wave theory (which is not affected by “kinematic” inconsistencies) and a calculation of the self-energy in the T-matrix formalism. At low temperature both theories are found to give a small renormalization of the energy gap in agreement with neutron scattering experiments. On the other hand the two approximations give significantly different renormalization at higher temperature. Measurements in this temperature range should be able to put in evidence non linear effects in the anisotropy.