Site Preference And Magnetic Properties Of Some Quaternary Heusler Alloys

This thesis mainly explores four series of quaternary Heusler alloys including Fe Ru YZ(Y=Ti,V,Cr,Mn and Z=Al,Ga,In,Si,Ge,Sn),Cr-doped Fe₂VSi,Fe₂V_1-xRu_xSi(x=0.25,0.5,0.75,1.00)and Ru-doped Mn-Ni-In.The site preference of transition metal elements,electronic structure and magnetic properties have been studied by theoretical calculations and experimental measurements.A series of quaternary Heusler alloys Fe Ru YZ(Y=Ti,V,Cr,Mn and Z=Al,Ga,In,Si,Ge,Sn)with 24-26 valence electrons have been systematically studied.The formation energy E_f of Fe Ru YZ increases with increasing atomic number of Z.The 24-electron Fe Ru Ti Si(Ge,Sn)are all paramagnetic semiconductors with real band gap at the Fermi level.Ferromagnetism and high spin polarization ratio are observed in the 25-electron Heusler alloys,among which Fe Ru Cr Al is predicted to have a 100%spin polarization ratio.The 26-electron Fe Ru Cr Si is found to be a spin gapless semiconductor with total spin moment of 2?_B/f.u.Fe₂V_1-xCr_xSi(x=0.1-0.4)Heusler alloys were synthesized experimentally.A small amount of A15 secondary phase is observed in Fe₂V_0.6Cr_0.4Si alloy,indicating that the solid solubility of Cr in Fe₂VSi has been reached its maximum.The Curie temperature T_Cand saturation magnetization M_s of Fe₂V_1-xCr_xSi both increase with increasing Cr content.In Fe₂V_1-xCr_xSi(x=0.25,0.5,0.75,1.00)Heusler alloys,half-metallic character is observed when x=0.5,0.75 and 1.00.When x is larger than 0.20/0.25,the experimental/theoretical total moment gets close to the Slater-Pauling curve of M=Z-24.This agrees with the predicted half-metallic character in these alloys.The site preference,electronic structure and magnetic properties of Fe₂V_1-xRu_xSi(x=0.25,0.5,0.75,1.00)Heusler alloys have been investigated theoretically and experimentally.First-principles calculations and experiments show that although Fe and Ru both have eight valence electrons,Ru tends to occupy A,C sites in the lattice.A typical ferromagnetic character is observed when x is larger than 0.25,so doping Ru to antiferromagnetic Fe₂VSi can be a possible way to realize ferromagnetism in this alloy.These results can help to understand the site preference of transition metal elements in Heusler alloys.A series of Mn₂Ni_1.6-xRu_xIn_0.4(x=0.1-0.5)Heusler alloys were prepared experimentally.When x=0.1-0.3,the martensitic transformation can be observed in these alloys.And as the Ru content increases,both the martensitic transformation temperature and saturation magnetization of the martensite phase decrease.When x=0.4and 0.5,the martensitic transformation disappears and these alloys are cubic austenite till low temperature.The change of phase transition temperature and the magnetic structure transition observed experimentally have been discussed by first-principles calculations.