The development of the magnetic excitation spectra of MnF2with increasing temperature

Abstract

The changes in the magnetic excitation spectra of MnF2 produced by increasing temperature in the range 0.75 TN to 1.2 TN have been studied by neutron inelastic scattering techniques. The discrete spin-wave spectra observed in a previous experiment at 4.2 °K by Low et al. suffer a line broadening and an energy decrease as the temperature is raised; also a continuous component appears. These three effects are small at about 50 °K (0.75 TN) but develop rapidly with increasing temperature. However, the total energy range of the spectra is seen to be independent of temperature up to the highest value used of 1.2 TN. Theoretical calculations of the effects of dynamical interactions between spin waves by Low are found to fit the observed dispersion changes within 10%, up to a temperature of about 62 °K (similar 0.9 TN). Beyond this temperature changes develop rapidly and well-defined spin waves are soon no longer recognizable in the observed spectra. At large wave vectors the spectra assume a continuous paramagnetic-like character just below the Neel point, and show no marked effects as the temperature is raised through the transition point. At small wave vectors abrupt critical effects are observed. Significant differences in the wave vector dependence of these effects observed near the 100 and 001 reciprocal lattice points demonstrate the effect of anisotropy in the transverse spin correlations. The observed value of the anisotropy parameter is in reasonable agreement (similar 30%) with the theoretical predictions of Moriya and by Tomita.