Spin dynamics of EuS in the paramagnetic phase

Abstract

The spin dynamics of the semiclassical Heisenberg model on the fcc lattice, with ferromagnetic interaction in the first-neighbor shell, antiferromagnetic interaction in the second-neighbor shell, and which undergoes a ferromagnetic transition, is studied in the paramagnetic phase at the temperature 1.1T_c using the Monte Carlo molecular-dynamics technique. The important quantities calculated are the dynamic structure function S(q,ω) and the spin autocorrelation function < S_i(0)·S_i(t) >. Our results for S(q,ω) show the existence of purely diffusive modes in the low-q regime. For q close to the zone boundary, our calculated S(q,ω) shows a two-peaked or a multipeaked structure depending upon the magnitude and direction of q and signifies damped propagating modes. This result disagrees with the theoretical predictions of Young and Shastry for all the principal directions and of Lindgard clearly for the < 100 > direction. Our results for S(q,ω) for q along the < 111 > direction is in fairly good agreement with the recent neutron scattering experiment of Böni et al.; however, our results for the < 100 > direction somewhat disagrees with the experiment of Bohn et al. Our calculated auto-correlation function shows a diffusive behavior temporally.