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

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.