The change of the symmetry of the soft-mode spin wave at the spin-flop transition as observed in MnTiO₃ using inelastic polarized neutron scattering

The energy and the eigenvector of the spin waves in MnTiO₃ have been studied as functions of the applied magnetic field using inelastic neutron scattering with and without polarization analysis. It is found that the behaviour depends on the distance q from the antiferromagnetic zone centre: for q ≅ 0 the mode softens completely at the spin-flop field H_sf, but it has finite energy for q > 0 the eigenvector retains its Ising character for small q up to the highest fields measured (5 T = 0.86H_sf, assuming H_sf = 5.8 T), but it changes continuously with field from Ising to Heisenberg character for large q. A very strong, temperature-dependent line width was found at H = 6 T, close to H_sf, which is interpreted as further evidence for the soft-mode instability at the spin-flop transition.