Unusual direction dependence of exchange energies in GaAs:Mn - Is the RKKY description relevant

TL;DR

This study challenges the RKKY model as an explanation for ferromagnetism in Mn-doped GaAs by revealing a strong directional dependence of exchange energies through first-principles calculations, suggesting alternative mechanisms like p-d hopping.

Contribution

The paper demonstrates that RKKY interactions cannot fully explain ferromagnetism in GaAs:Mn and highlights the importance of directional dependence and p-d hopping interactions.

Findings

Exchange energies show strong <110> and <100> directional dependence.

RKKY model is insufficient to explain observed ferromagnetism.

p-d hopping interactions are key to stabilizing ferromagnetism.

Abstract

Ferromagnetism in Mn-doped GaAs, the prototypical dilute magnetic semiconductor, has so far been attributed to hole mediated RKKY-type interactions. First-principles calculations reveal a strong direction dependence of the ferromagnetic (FM) stabilization energy of two magnetic ions, a dependence that cannot be explained within RKKY. In the limit of host-like hole (engineered here by an GGA+U approach with large U) where the RKKY model is applicable, we find that the exchange energies are strongly reduced, suggesting that this limit cannot explain the observed ferromagnetism. The dominant contribution stabilizing the FM state is found to be maximal for <110>-oriented pairs and minimal for <100> oriented pairs, providing an alternate explanation for magnetism in such materials in terms of energy lowering due to p-d hopping interactions, and offering a new design degree of freedom to enhance FM.