First-principles Study Of Heusler Alloys And Its Film Materials

Spintronics is a research hotspot in the field of condensed matter physics and materials science in recent years.How to inject polarized electrons into semiconductor materials efficiently is a key issue.Theoretically,it has been proved that the low polarization of magnetic metal materials is one of the reasons for the low spin injection rate.Semi-metallic magnetic materials are ideal semiconductor spintronic injection sources because of their100%spin polarizability,which makes them used widely in spintronic devices.Therefore,the theoretical and experimental search for materials with high spin polarization and in-depth study will promote the development of spintronics theory and its applications.Heusler alloy Co₂FeAl and Co₂Fe Si have been proven to be good semi-metallic materials with the 100%spin polarization in theory and experiment.This paper is based on first-principles density functional theory methods.Theoretical studies have predicted the physical properties of Co₂FeAl_1-xSi_x series alloys in which Si atoms replace Al atoms in Co₂FeAl alloys at different substitution concentrations,such as electronic structure,thermoelectric properties,tetragonal distortion,phonon spectrum and elastic constant.The electronic structure,tetragonal phase transition and magnetic properties of the corresponding thin film materials were also simulated.The calculations proves that:(1)The bulk phase of the Co₂FeAl_1-xSi_x series alloys are all Cu₂Mn Al type(L21 type),and the energy is the lowest in the ferromagnetic state,the Co₂FeAl_1-xSi_x series alloy of Cu₂Mn Al type is the most stable state under ferromagnetic conditions.(2)The Co₂FeAl_1-xSi_x(x=0.25,0.5,0.75)series alloy are all half-metal,the band structure has a band gap near the Fermi level,and in the spin downward direction which showing 100%spin polarization rate.The down spin states(semiconducting character)all exhibit good thermoelectric properties,and the power factor increases with the increased substitution concentration of Si atoms.At 300K,the doping concentrations x=0.25,0.5,and0.75 correspond to power factors of 28.70(?Wcm^-1K^-2),30.07(?Wcm^-1K^-2),30.74(?Wcm^-1K^-2).The calculated phonon spectrum does not have virtual frequency,indicating its dynamic stability,and all cubic phases fulfill the mechanical stability criteria,i.e Born criteria:C₁₁>0,C44>0,C₁₁-C₁₂>0,C₁₁+2C₁₂>0,and C₁₂<B<C₁₁.The calculation of specific heat capacity is consistent with Dulong-Petit's law and good stability of the cubic phase proves that there is no martensitic transformation.(3)For the Co₂FeAl_1-xSi_x(x=0.25,0.5,0.75)series alloy thin films all show high spin polarization.When the thicknesses were 1.6956nm?1.6982nm and 1.6922nm,the spin spin polarization the spin polarization is 43.09%,59.07%,100%,the film at x=0.75 was semi-metal.The lowest energy point of the structure is at c/a=1.2 when x=0.75,suggesting the martensitic transformation in this structure.With the variation of the tetragonal distortion,the total magnetic moment also changes and it is mainly determined by the change of atomic magnetic moment of transition-metals Fe and Co.When the film thicknesses were2.826nm?2.830nm and 2.820nm,the spin polarization were 100%,74.23%,and 45.73%,and the film at x=0.25 was semi-metal.The thickness of the film has a certain influence on the spin polarization of the material.(4)The Co₂FeAl_1-xGa_x(x=0.25,0.5,0.75)series alloys are all half-metallic with 100%spin polarization and clear band gap in the down spin direction.The phonon spectrum calculation results have no virtual frequency,and satisfy the dynamic stability condition.All cubic phases fulfill the mechanical stability criteria,i.e Born criteria:C₁₁>0,C44>0,C₁₁-C₁₂>0,C₁₁+2C₁₂>0,and C₁₂<B<C₁₁.The calculation of specific heat capacity is consistent with Dulong-Petit's law and there is no martensitic transformation.