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

Abstract

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 ${T}_{C}$ 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 $(\ensuremath{\sim}1\text{ }\text{ps})$ extending up to 15 atomic shells. Above $\ensuremath{\sim}10\text{ }\text{ps}$ 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.