Longitudinal spin fluctuations in the antiferromagnet MnF2 studied by polarized neutron scattering

In neutron scattering experiments using polarization analysis we separated the spectra of transverse and longitudinal magnetic fluctuations in the anisotropic antiferromagnet MnF2. While transverse modes are related to single-magnon scattering, the longitudinal part is essentially due to two-magnon scattering. They were measured at T = 30 K and 50 K well below the Neel temperature T-N = 67 K. The dynamic magnetic response due to two-magnon creation or annihilation is separated by a gap centered near the spin-wave frequency from the central peak corresponding to neutron-magnon scattering (creation of one magnon and annihilation of another). The longitudinal energy spectrum extends to about twice the frequency of the zone boundary modes. This tail at high energies is fairly independent of momentum transfer. The observed longitudinal spectra are in qualitative agreement with the theory for two-magnon processes and are determined for large energy transfers omega by the density of states D (omega/2)