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 s tudied as functions of the applied magnetic field using inelastic neut ron scattering with and without polarization analysis. It is found tha t the behaviour depends on the distance q from the antiferromagnetic z one centre: for q congruent to 0 the mode softens completely at the sp in-flop field H-sf, but it has finite energy for q > 0; the eigenvecto r retains its Ising character for small q up to the highest fields mea sured (5 T = 0.86H(sf), assuming H-sf = 5.8 T), but it changes continu ously with held 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 ins tability at the spin-flop transition.