Structures And Properties Of Graphene Decorated By Transition Metal Atoms And Clusters

Graphene,sp2 hybridized carbon atoms arranged in a honeycomb close crystal structure,has potential applications in spintronics and magnetic storage because of its unusual crystal structure and energy band structure.The one-dimensional graphene nanoribbon has the unique transport properties because its electrons are confined in the transverse and gap is opened.However,the perfect graphene is not magnetic,while the magnetic moment of per C atom in one-dimensional nanoribbon has only 0.3 ?B.In order to realize the application in magnetic storage and spintronics for graphene and nanoribbon,it needs to dope or adsorb the magnetic impurities on the graphene.The transition metal(TM)atom and TM clusters with strong spin-orbit coupling(SOC)are very promising materials in high density magnetic recording material because their large magnetic anisotropy energy(MAE).The MAE of the hybrid system still kept large due to the unique electronic structure of graphene.On the other hand,the graphene nanoribbons(GNRs)decorated by magntic atom and magnetic clusters have practical application in the spin transport because of inceasing magnetic moment.In this paper,the structure,magnetic and transport properties of the graphene and GNRs decorated by TM atom and clusters were studied using the first principle based on density functional theory and the non equilibrium Green function combined with the first principle.The main research contents are as follows:By using density functional theory,the structure,electronic and magnetic properties of divacancies(DV)and nitrogenized-divacancies(NDV)graphene decorated by TM atoms were investigated.Meanwhile the electric-field-manipulated magnetic properties have also systematically discussed.The results indicate that the DV-graphene and NDV-graphene decorated by TM atoms possess high stability due to the defect site can firmly fix TM on the surface of graphene.We also indicate that the Ir@DV-graphene,W@NDV-graphene,Os@NDV-graphene and Re@NDV-graphene have very high MAE with 20.8 meV,21.2 meV,27.5 meV and 71.2 meV,repectively.Combined with the density of states and the two order perturbation formulas,we illuminate that the electronic distribution of Fermi surface determines the magnitude of the MAE and the easy magnetization direction.Take W@NDV-graphene for example,we studied the manipulation of electric filed to magntic properties.More interestingly,both the magnetization direction and the magnitude of MAE for the W@NDV-graphene can be facilely manipulated by external electric fields.Based on the analyses on the projected density of states of W-5d orbitals and the change in tensile stain and orbital magnetic anisotropy,we give an insight on the origin of manipulating MAE by external electric fields for such kind of hybrid system.This work presents feasible strategies to realize high-performance magnetic storage and multi-state logical switching with ultralow energy consumption at sub-nanometer scale.We studied the structure,electronic and magnetic properties of the 4d-transition-metal-doped Mg8 clusters using spin-polarized density functional theory method.We demonstrate that TcMg8 is highly energetically stable in both structure and magnetic states,and identify it as a magnetic superatom with a magnetic moment as large as 5 ?B.The magnetic TcMg8 with twenty-three valence electrons has a configuration of 1S21P61D10 closed shell and 2S12D4 open shell,complying with Hund's rule similar to the single atom.We elucidate the formation mechanism of the magnetic TcMg8 superatom based on the detailed analysis of molecular orbitals,and attribute it to the large exchange interaction and moderate crystal field effects.In additon,we predict that the magnetic TcMg8 may exhibit semiconductor-like property with spin polarization characteristics.Finally,we investigated the transport properties of ZGNR decorated by Tc atom and TcMg8 using the non equilibrium Green function combined with the first principle.We found that the decorated Tc atom slightly reduces the current and increases the magnetic resistance of ZGNR in 0.4V,icreasing from 818.7%to 9.038 × 103%.However,the adsorptive TcMg8 can improve the transport current and reduce the magnetoresistance effect of perfect ZGNR,meanwhile selectly increase the spin-up current.Based on the analysis of the transmission spectra,the spatial distribution of molecular projected self-consistent Hamiltonian(MPSH)of the Fermi energy level and the energy band structure of electrode,we elucidate that a single Tc atom decreases the current of ZGNR because of after scattering incudced by the presence of a localized state while the adsorbed TcMg8 results in the increase of current due to resonance transmission induced by TcMg8 quantum dot.The structure and magnetic properties of Irn+i and CoIrn clusters are studied by density functional theory.The calculation results indicate that the MAE of Irn+1 are quite large,especially for Ir2 and Ir4 cluster with 67.5 meV/atom and 40.26 meV/atom,respectively.Meanwhile,the Ir4 cluster possesses high stability.The doped Co atom does not modify the structure of host clusters,whereas modulates the magnitude of MAE and the direction of easy magnetization for host clusters.In particular,the calculated MAE of CoIr dimer was as large as 0.98 eV/atom.The wave function character of d orbitals and density of states of Irn+i and CoIrn clusters have also been calculated to understand the mechanisms determining the sign and magnitude of MAE.Finally,the stability and MAE of the Ir4 inserted N-doped granphene pore(Ir4@NTV-Graphene)have been investigated.The results indicate that the hybrid system has a high stability and large MAE with 23.68 meV/atom.The analysis of electronic structure indicates that the magnitude and the direction of the easy axis of MAE attribute to the distribution of electron occupation number near the Fermi level.Finally,the transport properties of zizag nanoribbons(ZGNR)and ZGNR decorated by Co atomic chain were studied by using non equilibrium Green function combined with density functional theory.The results indicate that the ZGNR of anti-parallel spin configuration has a certain switch ratio and a good spin filtering properties,and has a magnetic resistance as large as 2.0×10%in 0.2V.Whether to magnetic parallel or anti-parallel spin configuration,the adsorbed Co atomic chains can selectively suppress spin-down current,thus Co@ZGNR system has a good spin filtering properties.In addition,the magnetoresistance of Co@ZGNR system is larger than that of ZGNR except for these in 0.2V.Based on the analysis of the energy band structure,the local density of states and the transmission channel,the change of transport properties of ZGNR was derived from the variation of energy band structure of the electrode and the distribution of energy level in central region induced by the adsorption of Co atomic chain.The findings might be able to points out a way for the control of spin current.