Study Of Properties Of Half-metallic Ferrimagnetism Of Mn-based Heusler Alloys

Recently a new discipline, called spintronics has emerged where it is not only the electron charge but the electron spin that carries information. The information carrier of traditional electronic components such as diode and audion is electronic charge, but the electronic spin is not considered. Studies on the semiconductor spintronics indicate that the diluted magnetic semiconductors can be used to manage and store information by means of the electronic charge and spin. However, nowadays, the key to semiconductor spintronics is that the efficiency of spin-polarized electrons injected into semiconductors is very low, one needs to find some magnetic materials with high spin-polarization. A half-metallic magnet is a novel material with special bandstructures. That is, for electrons of one spin direction(called majority) it behaves as a metal, while for the opposite spin direction (minority) it has a gap at E_F like a semi-conductor or an insulator, resulting in electrons around E_F being 100% spin polarization. Therefore, the material has a promising potential for applications on spintronics device and will become the perfect spin injection fountain to semiconductor.The half metallic materials we know include Heusler structural materials, the rutile structural materials (CrO₂) and spinel structural materials (Fe₃O₄, ZnFe₂O₄) etc. Because of the high Curie temperature and the crystal structure and lattice matching compatible with zinc-biende semiconductors used industrially, the Heusler alloys have been taken as the ideal materials. With the help of the plane-wave pseudopotential (PWPP) method based on the density functional theory (DFT), we investigated the magnetism and the properties of the half-metal of the Mn-based Heusler alloys. The main results were summaried as follows:1. we have investigated the electronic structure and magnetism of the Heusler alloys Mn₂CoZ (Z = Al, Ga, Si, Ge) with a high-ordered structure. Our calculations indicated that the high-ordered structural Mn₂CoZ alloys are all ideal half-metallic ferrimagnets around their respective equilibrium lattice constant. They have a total magnetic moment of 2.0μ_B for Mn₂CoAl (Ga) and 3.0μ_B for Mn₂CoSi (Ge) per unit cell, respectively. The results are in good agreement with the Slater-Pauling rule. Upon expansion or compression of the lattice, the Fermi level is shifted as in a rigid-band model. We also found that these compounds maintain half-metallicity within a wide range of lattice constants between 5.4 and 5.9 (?).We hope that the present work may be helpful for the experimental efforts toward fabrication and study of these high-ordered structural Mn₂CoZ alloys.2. The electronic structure, magnetism and the properties of half-metal of Mn-based Heusler Mn₂YZ(Y=Sc, Ti, V, Cr, Mn, Fe, Co; Z=Al, Si) were investigated using Ab initio calculation. The results showed that these alloys adopt different structures: the compound Mn₂YZ (Y=Sc, Ti, V, Cr, Mn) in which the atomic number in a row of the Periodic Table of Y is lower than that of Mn adopt the conventional Cu₂MnAl-type structure while the material Mn₂YZ (Y=Fe, Co) in which the atomic number in a row of the Periodic Table of Y is higher than that of Mn adopt the new high-ordered Cu₂MnAl-type structure. With their respective structure, the alloys Mn₂YAl (Y= V, Cr, Fe, Co) and Mn₂YSi (Y= Sc, Ti, Mn, Fe, Co) are of true half-metallic ferrimagnetisms and the alloys Mn₂ScAl, Mn₂TiAl and Mn₂VSi exhibit weakened half-metallic properties. However, the compounds which have 24 valence electrons per unit cell show special properties: the Mn₃Al exhibit half-metallic fully-compensated ferrimagnets, whereas the molecular magnetic moment of the Mn₂CrSi alloy is zero and the spin moment of each atom disappears.The magnetic moment of Mn(C) and Mn(A) is antiferromagnetically aligned to that of Y in the Mn₂YZ (Y=Sc, Ti, V, Cr, Mn) compounds with Cu₂MnAl-type structure. But for the Mn₂YZ (Y= Fe, Co) alloys adopting Hg₂CuTi structure, the magnetic moment of Mn(A) is antiparallel to that of Mn(B) and Y. All these Mn-based alloys Mn₂YZ follow the Slater-Pauling rule M_H=N_V-24 with varying Y atom. However, the molecular magnetic moment M_H increases with increasing valence concentration only by increasing the magnetic moment of the Y atom, while the magnetic moments of Mn in each structure are little touched. The magnetic moment carried by the Mn and Y atoms is restricted by the Z atoms even though they carry a negligible magnetic moment and do not directly contribute to the magnetic properties. Further, when we substitute Si for Al, we find that it is increased for the width of the band gap and the HM gap and it is decreased for the atomic magnetic moment of transition element, but it do not destroy the half-metallicity of these alloys. The finding are supported by an analysis of the site resolved occupy of 3d states.We also investigated the sensitive of the half-metallicity to changes in lattice constants. It is found that the width of the band gap and the location of the Fermi level will vary with the change of the lattice constants in a certain range, without destroying the half-metallicity of these Mn₂YZ compounds. As we expand the lattice, each atomic spin moment of the alloys will be enhanced.3. The electronic structure and the magnetism of the Heusler alloys Cr₂MnZ which have 24 valence electrons per unit cell were investigated. The calculation shows that the Cr₂MnZ alloys adopting Cu₂MnAl-type L2₁ structure are ideal half-metal around their respective equilibrium lattice constant. The spin magnetic moment of Cr and Mn atoms are antiparallel and the compounds are ferrimagnets. The half-metallic properties of these four alloys can be maintained for a wide range of lattice constants: 5.7-6.1 (?) for Cr₂MnP, 5.8-5.9 A for Cr₂MnAs, 5.8-6.2(?) for Cr₂MnSbå’Œ5.9-6.4 (?) for Cr₂MnBi. We hope our work can present some theoretical foundation for synthesizing experimentally these half-metallic compounds.