The RKKY Coupling between Impurity Spins in Graphene Nanoflakes

TL;DR

This paper investigates how the RKKY interaction between magnetic impurities in graphene nanoflakes varies with geometry, doping, and impurity placement, revealing conditions for ferromagnetic or antiferromagnetic coupling.

Contribution

It provides a detailed analysis of RKKY coupling in graphene nanoflakes considering geometry, doping, and impurity placement, highlighting the role of first- and second-order mechanisms.

Findings

Doping can switch coupling from antiferromagnetic to ferromagnetic.

Edge form and impurity placement significantly influence RKKY energies.

Odd electron count can lead to ferromagnetic coupling via first-order effects.

Abstract

We calculate the indirect charge carrier mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities for two selected graphene nanoflakes containing four hexagonal rings in their structure, differing by their geometry. We describe the electronic structure of either charge-neutral or doped nanoflakes using the tight-binding approximation with the Hubbard term, which is treated within the molecular-field approximation. We find pronounced differences in the RKKY coupling energies, dependent on the placement of the pair of magnetic moments in the nanostructure and on the edge form. For an odd total number of electrons in the structure, we predict in some circumstances the existence of ferromagnetic coupling with leading first-order perturbational contribution, while for an even number of charge carriers the usual, second-order mechanism dominates. Therefore, doping of the nanoflake with a single charge carrier is found to be able to change the coupling from an antiferromagnetic to a ferromagnetic one for some geometries.