Magnetic interactions of substitutional Mn pairs in GaAs

TL;DR

This study uses a tight-binding model to analyze magnetic interactions of Mn pairs in GaAs, revealing how their orientation and separation influence ferromagnetic stability and acceptor level splitting, with implications for spintronic applications.

Contribution

It provides a detailed theoretical analysis of Mn pair magnetic interactions in GaAs, highlighting the dependence on pair orientation and separation, and clarifying the relationship between acceptor splitting and exchange interaction.

Findings

Ferromagnetic alignment is most stable along the <110> direction.

Acceptor level splitting varies significantly with pair orientation.

Exchange interaction correlates with impurity level distribution width.

Abstract

We employ a kinetic-exchange tight-binding model to calculate the magnetic interaction and anisotropy energies of a pair of substitutional Mn atoms in GaAs as a function of their separation distance and direction. We find that the most energetically stable configuration is usually one in which the spins are ferromagnetically aligned along the vector connecting the Mn atoms. The ferromagnetic configuration is characterized by a splitting of the topmost unoccupied acceptor levels, which is visible in scanning tunneling microscope studies when the pair is close to the surface and is strongly dependent on pair orientation. The largest acceptor splittings occur when the Mn pair is oriented along the <110> symmetry direction, and the smallest when they are oriented along <100>. We show explicitly that the acceptor splitting is not simply related to the effective exchange interaction between the Mn local moments. The exchange interaction constant is instead more directly related to the width of the distribution of all impurity levels -- occupied and unoccupied. When the Mn pair is at the (110) GaAs surface, both acceptor splitting and effective exchange interaction are very small except for the smallest possible Mn separation.