Dynamics of diluted magnetic semiconductors from atomistic spin-dynamics simulations: Mn-doped GaAs

The dynamical behavior of the magnetism of diluted magnetic semiconductors (DMSs) has been investigated by means of atomistic spin-dynamics simulations. The conclusions drawn from the study are argued to be general for DMS systems in the low-concentration limit, although all simulations are done for 5% Mn-doped GaAs with various concentrations of As antisite defects. The magnetization curve $M(T)$ and the Curie temperature TC have been calculated and are found to be in good correspondence to the results from Monte Carlo simulations and experiments. Furthermore, equilibrium and nonequilibrium behaviors of the magnetic pair-correlation function have been extracted. The dynamics of DMS systems reveals a substantial short-ranged magnetic order even at temperatures at or above the ordering temperature, with a nonvanishing pair-correlation function extending up to several atomic shells. For the high As antisite concentrations the simulations show a short-ranged antiferromagnetic coupling and a weakened long-ranged ferromagnetic coupling. For sufficiently large concentrations we do not observe any long-ranged ferromagnetic correlation. A typical dynamical response shows that starting from a random orientation of moments, the spin correlation develops very fast (~1 ps) extending up to 15 atomic shells. Above ~10 ps in the simulations, the pair correlation is observed to extend over some 40 atomic shells. The autocorrelation function has been calculated and compared with ferromagnets such as bcc Fe and spin-glass materials. We find no evidence in our simulations for a spin-glass behavior for any concentration of As antisites. Instead the magnetic response is better described as slow dynamics, at least when compared to that of a regular ferromagnet such as bcc Fe.