결함을 가진 나노물질의 자성에 대한 제일원리 연구

Abstract

We have explored the magnetic properties of hexagonal 2p systems, hexagonal boron nitride (h-BN) and graphene using the full potential linearized augmented plane wave (FLAPW) method. First of all, we have explored the origin of magnetism based on the hexagonal boron nitride (h-BN) systems. It has been observed that the N vacancy defect has no influence on the magnetic property of h-BN, whereas the B vacancy defect induced spin polarization in the nearest three N atoms. Both B and N adatom defect systems preserve close to semiconducting feature with a finite band gap. We have found that the Density of state (DOS) and the X-ray magnetic circular dichroism (XMCD) spectral shapes are strongly dependent on the defect type. Also, we have investigated the magnetic properties of a hexagonal boron nitride (h-BN) monolayer induced by a 0.5 monolayer of oxygen (O) and fluorine (F) adlayers. It has been observed that the F and O adlayers are adsorbed on the boron top site in the most stable structure. We find that both systems display ferromagnetic ground states. In F/BN, the calculated magnetic moments of F and N atoms are 0.18 and 0.44. Also, the band structure of F/BN is very close to half-metallic. In O/BN, the calculated magnetic moments of O and N atoms are 0.91 and 0.4. The band structure shows a metallic state. Interestingly, it has been found that the O/BN may show a ferromagnetic ground state far beyond room temperature. Second, we report the theoretical results that the carrier induced switching of magnetic interaction between two magnetic layers in Co(111)/Graphene/Ni(111) (Co/Gr/Ni) is predicted. The Co/Gr/Ni shows an antiferromagnetic (AFM) ground state when there are no external carriers. The antiferromagnetic interaction is still observed for hole carriers. However, the magnetic exchange interaction between Ni and Co layers can be manipulated in such a way as to change an AFM to a ferromagnetic (FM) state by injecting external electrons. Overall, we propose that a potential spin switching by external carriers or electric field can be realized in Co/Gr/Ni. Besides, the calculated DOS feature indicates that the Co/Gr/Ni system may manifest quite different transport properties when a bias voltage is applied. For instance, the current parallel to the film surface can be completely spin-polarized from minority spin-electrons. In contrast, the current perpendicular to the film surface will be positively spin polarized from majority spin- electrons.