Structure,Electronic And Magnetic Properties Of Bulk And Thin-films Of Heusler Alloys:a First-principles Study

Heusler alloys are half-metallic(HM)ferromagnets with 100%spin-polarization at the Fermi level EF,because their band structure is metallic in one of the two spin channels and semiconducting in the other spin channel.Heusler alloys have potential advantages in the fields of spintronics and magnetic electronics.They can be divided into half-Heusler alloys,full-Heusler alloys and quaternary Heusler alloys.In this paper,we investigate the structural,electronic and magnetic properties of bulk half-Heusler alloy RhMnSb,full-Heusler alloys CsYZ2(Y=V or Cr;Z=F,Cl,Br or I)and Ti2Fe1-xCoxSn(x=0.00,0.25,0.50,0.75 or 1.00)and quaternary Heusler alloys TiZrCoTi1-xPbx,TiZrIrZ(Z=Al,Ga or In)and CoFeCrAl by first-principles calculations based on density functional theory(DFT).In recent years,the thin-film of the Heusler alloys have been investigated extensively.Some thin-films of the Heusler alloys have been found to still maintain the HM character,but the others lose the HM character.Therefore,we also study the structural,electronic and magnetic properties of various terminated thin-films of the Ti2Fe1-xCoxSn,TiZrIrZ(Z=Al,Ga or In)and CoFeCrAl alloys.The main innovative results are:(1)We investegate the structural,thermal,magnetic,mechanical and electronic properties of the RhMnSb half-Heusler alloy.Our findings reveal that the spin-dependent gap in the RhMnSb half-Heusler alloy can be controlled by tuning the lattice geometry through the application of pressure.The RhMnSb half-Heusler alloy makes a phase transition from a metallic to a HM phase when the external hydrostatic pressure exceeds about 21 GPa.Additionally,the spin-down channel can be further moved away from the Fermi level EF via gradually increasing the external pressure beyond the critical value which expands its importance in the spintronics applications at room temperature.Further,we found that RhMnSb half-Heusler alloy lose its mechanical stability when the external pressure exceeds 38 GPa.While at the same pressure,our phonon calculations show that it is still dynamically stable.Remarkably,we observe that the highest peaks of the absorption coefficient are detected in the ultraviolet(UV)region,indicating that the material is suitable for use in UV photodetectors and UV filters.(2)As for spintronic applications,all information is encoded in the spin degree of freedom,so the key issues are the high SP,the large HM bandgap,large magnetic moment and higher Curie temperature than room temperature.Our study reveals that all the halogen based CsYZ2(Y=V or Cr;Z=F,Cl,Br or I)full-Heusler alloys are complete 100%spin-polarization at the Fermi level EF.The electronic structures indicate that these alloys open very large bandgaps in the spin-down channel while the spin-up channel is metallic.Following the Slater-Pauling rule Mt=Zt-16,these alloys exhibit very large total magnetic moments Mt of 4.00 ?B/f.u.when Y=V and 5.00 ?B/f.u.when Y=Cr.The high Curie temperatures indicate that these alloys are appropriate for spintronic applications at room temperature.The HM character of these alloys is robust against external hydrostatic pressure.Furthermore,the highest peaks of the absorption coefficient are located in the UV region,indicating these full-Heusler alloys are also suitable for UV photodetectors and UV filters.(3)Thin-films with stable HM character and 100%spin-polarization are required to be used in spintronic devices.The HM character has been predicted theoretically in many Heusler alloys thin-films and confirmed by experiments.Full-Heusler alloy Ti2FeSn has been studied extensively.It has been reported that their(001)-oriented thin-films with TiFe or TiSn terminations preserve 100%SP,but the HM character is unstable because the edge of the bandgap is closed to the Fermi level Ef.Therefore,we investigate the effects of the Co-doping on the structural,electronic and magnetic properties of the bulk full-Heusler alloys Ti2Fe1-xCoxSn(x=0.00,0.25,0.50,0.75 or 1.00)and their(001)-oriented thin-films.The bulk Ti2Fe1-xCoxSn(x=0.00,0.25,0.50,0.75 or 1.00)alloys are all HM ferromagnets.We investigate twelve possible terminations and show that five of them preserve HM character with 100%spin-polarization at the Fermi level EF,while in the remaining seven,surface states emerge in the spin-down channel at the Fermi level EF,significantly reducing their SP.The Co-doping significantly increases the stability of the TiSn slab,also increases its spin-down bandgap Eg and HM gap EgHM at x=0.50.The stable HM character makes it is a slab of maximum benefit in the applications of spintronic devices,especially in magnetic tunnel junctions.(4)We study the effects of the Pb doping on the structural,electronic,magnetic and mechanical properties of the TiZrCoTl1-xPbx(x=0.00,0.25,0.50,0.75,1.00)alloys.The results show that the lattice constant,the absolute value of the formation energy,cohesive energy and Curie temperature increase with increasing Pb concentration.The TiZrCoTl1-xPbx(x=0.00,0.25,0.50,0.75,1.00)alloys are all HM ferromagnets,and their total magnetic moments Mt satisfy the Slater-Pauling rule Mt=Zr-18,where Zt is the total valence electrons.In the spin-down channel,the Fermi level EF shifts upwards and bandgap increases with increasing Pb concentration.The TiZrCoTl1-xPbx(x=0.00,0.25,0.50,0.75,1.00)alloys are all anisotropic ductile and mechanical stable materials.(5)Inspired by the unique properties and potential applications especially in spintronics,here we investigate the structure and electronic properties of the bulk TiZrlrZ(Z=Al,Ga or In)quaternary Heusler alloys and their(001)-oriented thin-films.The results show that all the bulk TiZrlrZ alloys are HM ferromagnets with a total magnetic moment Mt of 2 ?B/f.u.,satisfying the Slater-Pauling rule Mt=Zt-18.Their spin-down bandgaps are resulted from the d-d hybridizations between the transition metal elements Ti,Zr and Ir.The large absolute values of both positive cohesive energies and negative formation energies indicate these three alloys are thermodynamically stable and can be fabricated experimentally.The nonappearances of negative phonon frequency demonstrate their dynamic stability.Furthermore,their(001)-oriented thin-films with natural ZrZ(Z=Al,Ga or In)terminations are still HM ferromagnets and thus useable in spintronics and magnetoelectronics fields.(6)We study the structure,magnetic and electronic properties of the quaternary Heusler alloy CoFeCrAl in bulk and the(001)-,(110)-and(111)-oriented thin-films.The results show that,for the CoFeCrAl(001)-oriented thin-films the HM character of the bulk CoFeCrAl alloy is maintained in the CrCr*slab,but lost in the CoFe,CoCo*and FeFe*slabs,the CrAl and AlAl*slabs are nearly half-metals.The AlAl*slab with the lowest surface energy is the most stable,while the CrAl and CrCr*slabs are the second and third most stable slabs,respectively.The HM character of the bulk CoFeCrAl alloy is lost in the natural CoFeCrAl slab of the(110)-oriented thin-film and the natural CoCo,FeFe,CrCr and AlAl slabs of the(111)-oriented thin-films.