Title: Strain-Tunable Spin Moment in Ni-Doped Graphene
Author(s): Santos E.J.G., Ayuela A., Sánchez-Portal D.
Journal Physical Chemistry C, 116, No. 1, pp. 1174-1178 (29 November 2012)
Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here, we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Nisub). We have found that, although Nisub defects are nonmagnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3–0.4 μB per Nisub, until a critical strain of ∼6.5% is reached. At this point, a sharp transition to a high-spin state (∼1.9 μB) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene.
Title: Magnetism of Single Vacancies in Rippled Graphene
Author(s): Santos E.J.G., Riikonen S., Sánchez-Portal D., Ayuela A.
The Journal of Physical Chemistry C, 116, No. 13, pp. 7602-7606 (2 March 2012)
Using first-principles calculations, the dependence in the properties of the monovacancy of graphene under rippling controlled by an isotropic strain was determined, with a particular focus on spin moments. At zero strain, the vacancy shows a spin moment of 1.5 μB that increases to ∼2 μB when the graphene is in tension. The changes are more dramatic under compression in that the vacancy becomes nonmagnetic when graphene is compressed more than 2%. This transition is linked to the structural changes that occur around vacancies and is associated with formation of ripples. For compressions slightly greater than 3%, this rippling leads to formation of a heavily reconstructed vacancy structure consisting of two deformed hexagons and pentagons. Our results suggest that any magnetism induced by vacancies that occurs in graphene can be controlled by applying strain.
Title: Universal magnetic properties of sp3-type defects in covalently functionalized graphene
Author(s): Santos E.J.G., Ayuela A., Daniel Sánchez-Portal D.
New Journal of Physics, 14, No. 4, pp. 043022- (19 April 2012)
Using density-functional calculations, we study the effect of sp3-type defects created by different covalent functionalizations on the electronic and magnetic properties of graphene. We find that the induced magnetic properties are universal, in the sense that they are largely independent of the particular adsorbates considered. When a weakly polar single covalent bond is established with the layer, a local spin moment of 1.0 μB always appears in graphene. This effect is similar to that of H adsorption, which saturates one pz orbital in the carbon layer. The magnetic couplings between the adsorbates show a strong dependence on the graphene sublattice of chemisorption. Molecules adsorbed at the same sublattice couple ferromagnetically, with an exchange interaction that decays very slowly with distance, while no magnetism is found for adsorbates at opposite sublattices. Similar magnetic properties are obtained if several pz orbitals are saturated simultaneously by the adsorption of a large molecule. These results might open new routes to engineering the magnetic properties of graphene derivatives by chemical means.