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

Abstract

The dynamical behavior of the magnetism of diluted magnetic semiconductors ͑DMSs͒ 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% Mndoped GaAs with various concentrations of As antisite defects. The magnetization curve M͑T͒ 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 ͑ϳ1 ps͒ extending up to 15 atomic shells. Above ϳ10 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.

Equation of state of paramagnetic CrN fromab initiomolecular dynamics

“…As we pointed out above, the latter was estimated to be of the order of 20-50 fs in bcc Fe above T C . 27 For CrN, with a T N around room temperature and probably with weaker exchange interactions, we expect that t dc could be somewhat larger.…”

Recent progress in simulations of the paramagnetic state of magnetic materials

“…The relevant time scale can therefore be better estimated from the spin decoherence time t dc rather than from the inverse spin-wave frequency. For body-centered cubic (bcc) Fe above T C t dc is of the order of 20-50 fs [11]. Thus, the magnetic degrees of freedom in the paramagnetic state should be considered as much faster in comparison to those in magnetically ordered materials, of the order of 10 À14 -10 À15 s. Moreover, it is important to realize that the paramagnetic state is the high-temperature state of a magnetic material.…”

“…First principles calculations in the framework of Density Functional Theory (DFT) [4] have been recognized in the field as an extremely useful research tool. With a development of efficient computational methods [5] and increasing power of modern computers calculations of key magnetic characteristics, http like the local magnetic moments or magnetic exchange interactions, have transformed into a routine task, and theoretical treatments of noncollinear spin configurations [6,7], magnetic phase transitions [8], and spin dynamics [9][10][11][12][13] have become possible.…”

Third-order many-body perturbation theory applied to Kondo-type dynamics in diluted magnetic semiconductors

“…Contrary to the model of Ref. 12, in the present work the dynamical effects under a strong laser excitation are carefully reproduced, by allowing the time evolution of the ion-hole spin correlations and the off-diagonal components of the electronic density matrix to take place at the same time scale.…”

“…In Ref. 12, the magnetism evolution in a DMS has been investigated by means of atomistic spin-dynamics simulations where the interatomic exchange interaction is calculated selfconsistently. In analogy with the Born-Oppenheimer approximation, this procedure relies on the adiabatic approxi-mation.…”