Bias induced ferromagnetism and half-metallicity in graphene nano-ribbons

Rita Maji,Joydeep Bhattacharjee

doi:10.1038/s41598-017-17091-w

Abstract

Towards spin selective electronics made of three coordinated carbon atoms, here we computationally propose robust and reversibly bias driven evolution of pristine undoped graphene nano-ribbons(GNR) into ferromagnetic-semiconductor, metal or a half metal, irrespective of their edge configurations. The evolution is a result of a rare ferromagnetic(FM) order emerging among nearest neighbouring(n-n) sites, in positively biased regions in their in-homogeneous bias unit-cells, in attempt to cooperatively minimise on-site Coulomb repulsion and kinetic energy, while maximising localization of electrons at the positively biased sites. The phenomenon appears to be a general property of in-homogeneously biased Coulomb correlated bipartite systems. Consequences are particularly rich in zigzag edged graphene nano-ribbons(ZGNR) due to the contest of bias driven n-n FM order and the inter-edge antiferromagnetic order inherent to ZGNRs, leading to systematic closing of gap for one of the spins, amounting to bias controlled unmissable opening of window for FM-semiconducting and half-metallic transport.

Highlights

Sheets, ribbons and tubes made of three coordinated sp[2] hybridized carbon(C) atoms can be semiconducting or metallic[1,2], depending on their shape, size and edge configuration
Description of magnetism sourced at Coulomb correlation among itinerant electrons, as derived within the framework of Hubbard model[36] suggests primarily two classes of mechanisms to rationalize FMn −n ordering in bipartite systems upon deviation from half-filling[37]
Central to this approach is the robust emergence of FMn −n in the positively biased regions of the unit-cell, which we argue below to be a general property of in-homogeneously biased bipartite systems, arising primarily as a means to avoid increase of on-site Coulomb repulsion and kinetic energy while maximizing response to the external bias

Summary

Introduction

Ribbons and tubes made of three coordinated sp[2] hybridized carbon(C) atoms can be semiconducting or metallic[1,2], depending on their shape, size and edge configuration. Based on mean-field and ab-initio computation of spin resolved electronic structure, in this work we suggests an alternate approach to manipulate magnetism in graphene nano-ribbons(GNR), wherein, any GNR irrespective of its edge configuration, can be controllably as well as reversibly, turned into a FM semiconductor or metal and a half-metal, exclusively through spatially in-homogeneous biasing Central to this approach is the robust emergence of FMn −n in the positively biased regions of the unit-cell, which we argue below to be a general property of in-homogeneously biased bipartite systems, arising primarily as a means to avoid increase of on-site Coulomb repulsion and kinetic energy while maximizing response to the external bias. We demonstrate in the following the onset of FMn −n accompanied by lifting of spin degeneracy and consequent opening of window for ferromagnetic and half-metallic transport, in a representative variety of AGNRs, ZGNRs and in a minimal model system

Methods

Results

Conclusion