Density Functional Theory Studies On The Electronic Structures And Magnetism Of Several Low-dimensional Spintronics Materials

Spintronics,the emerging technology of electronic spin effect combined with traditional electronics,provides the unprecedented opportunities for the research and development of the next generation of microelectronic devices.Traditional semiconductor material is based on the electronic motion which speed will be subjected to the restriction of energy dispersion,and while the spintronics devices from the directional change and the coupling of electronic spin can realize the change of millions of times of logical state per second.Compared to the traditional semiconductor devices,spintronics devices remarkably reduce the consumption of energy,that is,spintronics devices consume lower energy to achieve faster and richer performance.(i)Graphene with a two-dimensional crystal constructed as sp2 hybridization with single atomic layer thickness has attracted extensive interest in the scientific community owing to excellent properties in mechanical,thermal,optical and electronic natures.Based on density functional theory(DFT),the electronic structure and magnetic manipulation in metal atoms adsorbed on trilayer graphene,as well as hydrogen atom and hydrogen dimer adsorbed on single layer and bilayer graphene are mainly discussed in the paper.(ii)Half-metal(HM)material with as high as 100% spin-polarization of conduction electrons is a kind of spintronics material with extremely important scientific significance and applicant prospect.Based on DFT,it is focused on the structural stability,electronic structure and magnetic properties of the(001)surface state in Heusler half-metal alloy Cr2 Co Ga in the final paper.Firstly,using DFT(VASP),we investigate the electronic and magnetic properties of an adatom(Na,Cu and Fe)on ABA-and ABC-stacked(Bernal and rhombohedral)trilayer graphenes.Particularly,we study the influence on magnetism in an applied gate voltage which modifies the electronic states of the trilayer graphene(TLG)and changes the adatom spin states.Our study performed for the choice of three different adatoms(Na,Cu and Fe)shows that the nature of adatom-graphene bonding evolves from ionic to covalent bond in moving from an alkali metal Na to a transition metal of Cu or Fe.Application of an external electric field(EEF)to trilayer graphene system with different stacking orders results in the transition between high-and low-spin states in the latter case(Cu and Fe)and induces a little of magnetism in the former(Na)without magnetism in the absence of external electric field.The results from our study will be valuable in references and significant in guidelines for the development of spintronics.Secondly,DFT(VASP)was used to investigate the electronic and magnetic properties of hydrogen atom and dimer chemisorbed on graphene single layer(SL)and bilayer(BL).Here we demonstrate that the adsorption of hydrogen dimer on single layer and bilayer graphenes symmetric ?? sites in different sublattice induces magnetic moments characterized by ferromagnetism(FM)spin-split states at the Fermi energy and while the adsorption on asymmetric ?? sites at the same sublattice switches off the magnetism and exhibits antiferromagnetism(AFM).Moreover,our scanning tunneling microscopy(STM)simulations complemented by DFT calculations show that such spin-polarized states are mainly localized on the carbon sublattice region opposite to the one where the hydrogen dimer are chemisorbed.These atomically tuned spin textures,which extends several angstroms away from the hydrogen dimer,drive the direct and strong coupling between the magnetic moments at unusually long distance.By manipulating the adsorption sites of hydrogen dimer with atomic precision,it is possible to cleverly tailor the magnetism of the selected graphene regions.Finally,half-metallic fully-compensated ferrrimagnets(HM-FCFs)are important spintronics materials due to the high spin polarization and the low magnetic moment.Ordered inverse Heusler structural Cr2 Co Ga thin films were grown by molecular beam epitaxy(MBE),and the predicted low magnetic moment and high Curie temperature were also confirmed.In order to compare the electronic and magnetic properties between bulk and thin films for Cr2 Co Ga,we explore the structural stability,electronic and magnetic properties of XA-and L21-Cr2 Co Ga(001)surfaces by using first-principles calculations(WIEN2k).(i)For the XA-Cr2 Co Ga(001)surfaces,it is found that the nearly half-metallicity of bulk XA-Cr2 Co Ga is preserved at the Cr2Ga-terminated(001)surface,and the spin polarization is a little increased compared to the bulk.However,the Cr1Co-terminated(001)surface destroys the bulk nearly half-metallicity due to the majority-spin surface states at the Fermi level.Cr atomic magnetic moments at both(001)surfaces are greatly increased compared to those in bulk for Cr2 Co Ga.In addition,we reveal that the Cr2Ga-terminated(001)surface is more stable than the Cr1Co-terminated(001)surface over the whole effective chemical potential.(ii)For the L21-Cr2 Co Ga(001)surfaces,it is found that the half-metallicity of Cr1Cr2-terminated(001)surface is obviously increased(better than that of Cr2Ga-terminated for XA),and the spin-polarization is up to 92%(much higher that of69% in bulk).While the half-metallicity of Co Ga-terminated(001)surface with only 27%spin-polarization has completely lost,the atomic magntic moments belonging to Co and Ga are hardly changed.Moreover,there is even better stability for the Cr1Cr2-terminated(001)surface over the whole effective chemical potential.These studies indicate that ordered inverse Heusler structural Cr2 Co Ga thin films are promising candidates for spintronics applications.