A critical analysis of vacancy-induced magnetism in mono and bilayer graphene

TL;DR

This study uses density functional theory to show that vacancy-induced magnetic moments in graphene vanish at realistic vacancy concentrations, challenging previous claims of vacancy-induced magnetic order.

Contribution

It provides a comprehensive analysis demonstrating that extended magnetic moments from vacancies do not sustain magnetic order in graphene at typical concentrations.

Findings

Vacancy-induced magnetic moments vanish at realistic concentrations.

Extended $ ext{pi}$ magnetic moments are not capable of long-range order.

Full $ ext{sigma}$-bond passivation can recover magnetic moments.

Abstract

The observation of intrinsic magnetic order in graphene and graphene-based materials relies on the formation of magnetic moments and a sufficiently strong mutual interaction. Vacancies are arguably considered the primary source of magnetic moments. Here we present an in-depth density functional theory study of the spin-resolved electronic structure of (monoatomic) vacancies in graphene and bilayer graphene. We use two different methodologies: supercell calculations with the SIESTA code and cluster-embedded calculations with the ALACANT package. Our results are conclusive: The vacancy-induced extended $\pi$ magnetic moments, which present long-range interactions and are capable of magnetic ordering, vanish at any experimentally relevant vacancy concentration. This holds for $\sigma$-bond passivated and un-passivated reconstructed vacancies, although, for the un-passivated ones, the disappearance of the $\pi$ magnetic moments is accompanied by a very large magnetic susceptibility. Only for the unlikely case of a full $\sigma$-bond passivation, preventing the reconstruction of the vacancy, a full value of 1$\mu_B$ for the $\pi$ extended magnetic moment is recovered for both mono and bilayer cases. Our results put on hold claims of vacancy-induced ferromagnetic or antiferromagnetic order in graphene-based systems, while still leaving the door open to $\sigma$-type paramagnetism.