CRITICAL AND PARAMAGNETIC SPIN DYNAMICS IN AN ANTIFERROMAGNETICALLY COUPLED HEISENBERG MAGNET - RESULTS FOR RBMNF3

Results are provided for the spin-spin response function of a three-dimensional, antiferromagnetically coupled Heisenberg magnet covering a range of temperatures from the critical temperature to deep in the paramagnetic phase. The wave vectors considered span the Brillouin zone. Damping rates are given at the zone centre and the antiferromagnetic-ordering wave vector, w, together with copious numerical results for the full response function. The calculations are based an the non-linear, integral-differential equations obtained from so-called coupled-mode theory. In a confrontation between experimental and theoretical findings for RbMnF3 nearly all aspects have a positive outcome. The main exception is found at T-c for wave vectors close to w. Here, the measured response comprises three distinct components, reasonably ascribed to diffusive and oscillatory collective processes. In the corresponding predictions, the diffusive component is conspicuously missing. A less pronounced discrepancy if found at the antiferromagnetic zone boundary where, once again, there is more structure in the observed spectrum than in the calculated one.