The Structure And Magnetism Of Nano-structured W And TM Doped DMS

The studies on the magnetism of materials,especially in the nano-structured scale,have been an important field,because of their very important applications in electronics and other realm of science.The nano-magnetism has become a stirring region of study because of their potential important applications in future semiconductors and highly integrated devices.Also,the spintronics,with the study of spontaneously employing the charge and spin of an electron in some bulk semiconductor materials,has become a very attractive research realm.Theoretically, the investigation of the relationship between the structure and the magnetism can also reveal some basic physical principles and discover important potential applications. The development of density functional theory(DFT) has opened a wide realm for the studies of electronic structures,and provided a pivotal scheme.In this dissertation,by using the first-principles calculations based on the density functional theory and by introducing the non-collinear calculations with the spin-orbital coupling,the structural stabilities,electronic structures and magnetic properties of zero-dimensional W clusters,one-dimensional W atomic chain,two-dimensional W atomic sheet and some TM-doped semiconductor materials have been systematically studied.As a result, some valuable information has been obtained and some important results have been achieved,i.e.:(1) The structural properties of W_n clusters(n = 3～27) has been studied.The most stable structures of clusters(n = 3～7) with global energy minimum and typical structures of clusters(n = 8～27) with local energy minimum are determined by the ab initio calculations of optimizing the structures.Based on the jellium model,the electronic configuration is proposed which can explain well the electronic magic numbers and the relative stabilities of W clusters.The binding energies,the first and second differences of binding energies and the HOMO-LUMO gaps versus the number of atoms in the clusters are obtained and analyzed.The character of the variety of the properties of bonding between the atoms versus the number of atoms in the cluster has been obtained,indicating that W clusters become metallic very quickly with the increase of cluster size.(2) The magnetism of free standing W atomic chains is studied.The calculations have been performed on the diversification of structural stability and the magnetism of ferromagnetic,anti-ferromagnetic and spiral polarized(spin wave) states.The chains with and without the spin-orbit(L-S) coupling are taken into account, respectively.It is shown that the stable W atomic chains are magnetic,and the anti-ferromagnetic is shown to be the most stable one.There is no magnetism in the chains when the atomic distance is small,however,the magnetic moment increases rapidly within a small region of atomic distance when the distance is larger than a certain value,and then approaches the value of a single atom.The orbital magnetic moment emerges when the inter-atomic exchange interaction emerges,and the orbital polarized direction is opposite to the spin polarized one.The stable anti-ferromagnetic W chain is metallic besides magnetic,and the magnetic moment is primarily contributed by d-electrons.When the atomic distance increases,the chain transforms gradually from metal to semiconductor,the atomic magnetic moment increases and the electrons are more localized,in the meantime,the s-electrons contributes more to the magnetism of the chain.(3) The structural and magnetic properties of two-dimensional W atomic sheet are studied.The calculations are performed for the plain structures with rhombic, hexagonal,rectangle and square and with variant lattice constants.The calculated magnetic structures include ferromagnetism,anti-ferromagnetism and partial anti-ferromagnetism.The structural stability,magnetism and electronic structure properties of different structures are compared.The main results are:the near-hexagonal structure(a structure with small distortion from hexagonal structure due to the John-Teller effect) is the most stable one in all the plain structures;the anti-ferromagnetism is more stable than ferromagnetism;the magnetism of some ferromagnetism structure appears oddity;the stable atomic sheets are nonmagnetic or with week magnetism;the stable square-structure appears week magnetism,and the transportation electrons also appears dilute polarization;the stable W atomic sheets are metallic.The magnetism appears in the sheets when the lattice constant is elongated,moreover,the magnetism appears more early in anti-ferromagnetism structures and more quickly in the square structure.The charge densities,the electronic density of states,and the magnetization density of hexagonal and square structures are also compared.The electronic structure properties and the effect of spin-orbit non-collinear coupling are discussed.(4) The magnetic properties ofⅢ-Ⅴsemiconductor(GaAs,GaP) doped by 3d-TM(TM=V,Cr,Mn,Fe,Co and Ni) are studied.It is shown that the ferromagnetic state(FM) will be realized in V,Cr and Mn doped GaAs and GaP;the antiferromagnetic(AFM) state is favored when doped by Fe,whereas,the materials show unstable magnetism when doped by Co and Ni.TheⅢ-Ⅴsemiconductor doped by Cr is a candidate with high Curie temperature(T_C).Furthermore,it shows that the magnetic moment of TM is larger than theoretical expected value when the spin-up t_2g-orbitals of TM are empty,whereas,the magnetic moment of TM become smaller than expected value when the spin-up t_2g-orbitals are fully occupied.When the spin-up t_2g-orbitals are partially occupied,the difference between TM's magnetic moment and expected value leans heavily on the crystal symmetry and the band structures of the magnetic ions.Finally,GaAs and GaP co-doped by Si and Mn are studied,it shows that co-doping will show better stability of FM state and higher T_C.