Role of Disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN

TL;DR

This study investigates how disorder affects magnetic interactions and critical temperature in Mn:GaAs, Cr:GaAs, and Cr:GaN, revealing that disorder and clustering significantly lower T_c and that exchange interactions decay exponentially with distance.

Contribution

The paper combines local spin density approximation with a linear-response technique and large supercell models to accurately analyze disorder effects on magnetic properties in these materials.

Findings

Disorder causes large variations in exchange interactions.

Disorder and clustering reduce the critical temperature T_c.

Exchange interactions decay exponentially with distance R^3.

Abstract

We present calculations of magnetic exchange interactions and critical temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonion, but no significant further approximations are made. Special quasi-random structures in large unit cells are used to accurately model the disorder. T_c is computed using both a spin-dynamics approach and the cluster variation method developed for the classical Heisenberg model. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T_c. Additionally the exchange interactions J(R) are found to decay exponentially with distance R^3 on average; and the mean-field approximation is found to be a very poor predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of spin-orbit coupling on T_c is considered. With all these factors taken into account, T_c is reasonably predicted by the local spin-density approximation in MnGaAs without the need to invoke compensation by donor impurities.