First-principles Study Of D~0Ferromagnetism In CdS And ZnS

In recent years, the d0magnetic materials with half-metllicity have been thepromising spin injection materials for spintronic devices. The d0magnet, in whichthere are no magnetic precipitates and second-phase particles, is different from thetraditional magnetic semicondoctors prepared by doping magnetic elements.Therefor, it is necessary to explore d0magnetism with half-metllicity via dopingnonmagnetic elements in nonmagnetic semiconductors. Based on the densityfunctional theory and previous study results made by other researchers, this work,united VASP ab-initio simulation package program, reveals the mechanismresponsible for d0magnetism in CdS-and ZnS-based materials.In chapter1, we review conceptions of spintronics and dilute magneticsemiconductor, expatiate on various mechanism of the magnetic coupling in magneticsemiconductor, and summarize the uptodate progression in ZnS-, CdS-and ZnO-basedd0magnetism.The second chapter briefly introduces the solution of the schroedinger equationwithin Born-Oppenheimer approximation, Hartree-Fock approximation andsingle-activeelectron approximation, and the approach to obtaining the electronicstructures based on the density functional theory.In the third chapter, the magnetic property induced by neutral Cd and S vacanciesin CdS bulk and thin film are investigated, using first principles simulation. For bulkCdS, the magnetism originates from Cd, instead of S, vacancies. The ferromagnetismwith a Curie temperature above room temperature can be expected. For CdS thin film,both Cd and S vacancies does not yield any magnetism at the (111) surface, whilelocal magnetic moments at the outermost S plane of the (001) surface is ascribed tothe surface effect. The ferromagnetism at S-terminated (001) surface can account forthe experimental observation. Also, the spherical CdS quantum dots with varioussizes show ferromagnetic behaviors, which is closely related to the surface statesassociated with the dangling bonds. The unsaturated bonds are responsible for thesurface magnetism.In the fouth chapter, based on ab-initio density functional theory calculationswithin the generalized gradient approximation, the electronic structure and magneticproperties of the one-and two-C-doped CdS monolayers are investigated. The results show that the C-doped CdS system exhibit half-metallic behaviors with a totalmagnetic moment of2.0μB/C, which is in good accord with the results obtained fromthe HSE06functional. The magnetic moment mainly comes from the spin-polarizedC-2p states in the band gap. The long-range ferromagnetic order with a Curietemperature of about280K is attributed to the p-d and p-p hybridizations via theC-Cd-S coupling chains. The N-doped CdS monolayer still is semiconductor and eachN atom give rise to a total magnetic moment of2.0μB. The couplinig interactionbetween the localized magnetic moments on N atoms is anti-ferromagnetic.In this chapter, the electronic structures and magnetic properties in zinc-blendestructure ZnS doped with nonmagnetic noble metal palladium have been investigatedby means of density functional theory (DFT) calculations employing the generalizedgradient approximation (GGA) and the GGA plus Hubbard U (GGA+U). Both theGGA and GGA+U methods demonstrate half-metallicity in Pd-doped ZnS with totalmagnetic moments of about2.0μBper supercell. The half-metallic ferromagnetismstems from the hybridization between Pd-4d and S-3p states and could be attributed toa double-exchange mechanism. These results suggest a recipe for obtaining apromising dilute magnetic semiconductor by doping nonmagnetic4d elements in ZnSmatrix.