First-principles Study Of Zr-based Heusler Alloys

Efficient spin injection from a ferromagnet to a semiconductor is very crucial for the improvement of the performance of spintronic devices,and thus the materials with high spin polarization,high Curie temperature Tc,and compatible lattice structure with conventional semiconductors are desired in practical spintronic applications.In particular,most magnetoelectronic devices rely on an imbalance in the number of majority and minority spin carriers,with the ideal material exhibiting a complete(100%)spin polarization at the Fermi level(EF).Up to now,a series of theoretical efforts was launched to predict a new half-metal or spin gapless semiconductor with effective Curie temperature Tc in Heusler alloy systems due to their useful applications in spintronic devices.Moreover,Heusler alloys exhibit rich physical properties and many applicable functions.In this dissertation,we will try to explore some new Zr-based Heusler alloys with high polarization by using first-principles projector-augmented wave(PAW)potential within the density functional theory(DFT)framework.Several conclusions are summarized as follows:(1)The electronic structures,magnetic properties,and half-metallicity of the ternary full-Heusler alloys Zr2CoZ(Z=Al,Ga,In,Sn)with the Hg2CuTi-type structure have been systematically studied.First,to confirm the magnetic ground state of these alloys,we calculate the total energy curves as a function of lattice constant in three different magnetic configurations:ferromagnetic(FM),antiferromagnetic(AFM),and nonmagnetic(NM)states.The results show that for all alloys,the FM state is the most favorable magnetic ground state.The electronic and magnetic calculations find the ZnCoZ(Z=Al,Ga,In,Sn)are half-metallic ferromagnets and kept within a certain range of the lattice constant.The total magnetic moments(?t)of the ZnCoZ alloys are calculated to be 2 ?B for Z=Al,Ga,In and 3 ?B for Z=Sn,linearly scaled with the total number of valence electrons(Zt)by ?t = Zt-18.The band gap in minority spin channel for these half-metallic alloys is determined by the lowest unoccupied 3 tu state and the highest occupied 3 t2g state.(2)The structural,electronic,magnetic and mechanical properties of the quaternary Heusler alloys ZrTiCrZ(Z=Al,Ga,In,Si,Ge,Sn)have been systematically studied.We first carry out the structural optimization and calculate the energy versus volume for three possible configurations for nonmagnetic(NM)and ferromagnetic(FM)phases for quaternary Heusler alloys ZrTiCrZ.The calculated results show that for all alloys,the most stable structure is Y-type ? configuration for FM phase.The equilibrium lattice constants also have been obtained by the structural optimization calculation.At their equilibrium lattice constants,the ZrTiCrZ alloys are half-metallic(HM)ferrimagnets for Z=Al,Ga and In,while spin-gapless semiconductor(SGS)antiferromagnets(AFM)for Z=Si,Ge and Sn.The total magnetic moments ?t of the ZrTiCrZ alloys are-1 ?B/f.u.for Z=Al,Ga and In,while 0 ?B/f.u.for Z=Si,Ge and Sn,both linearly scaled with the total number of valence electrons Zt by Slater-Pauling rule ?t = Zt-18.The elastic constants C11,C12 and C44 of the single crystal and the related elastic moduli G,B,E,v and A of the polycrystalline aggregates are also calculated and used to study the mechanical stability of these alloys.The Curie temperatures Tc of the ZrTiCrZ alloys are overestimated by using the mean field approximation(MFA).Finally,the HM stabilities as well as the total and atomic magnetic moments of the ZrTiCrZ alloys(Z=Al,Ga,In)under either hydrostatic strain or tetragonal strain are also discussed.