Effect Of Doping On Electroinc Structure And Magnetic Properties Of Half-Heusleralloys

In this thesis, The Ni₅₀Mn_36.5Sb_13.5-xB_x(x=0, 0.5, 1.0, 1.5) and theNi₅₀Mn_36.5Sb_13.5-xSi_x (x=0.5,1.0,1.5,2.0,2.5) alloys were prepared by arc-melting.Then XRD, VSM, SQUID DTA and SEM methods were used to test the samples.We studied the magnetic properties, structure and phase-transformation of thesealloys. Based on first-principles theory, we predicted the phase stability of Ni₂FeZ(In,Sn,Sb) alloys. And we also investigated the magnetic properties of these alloys.The electronic structure and magnetic properties of the main-group-element-richHeusler alloys CoYAl₂(Y=Cr, Mn, Fe) has been studied by ab-inito calculationsand new half- metallic materials have been discovered.Effect of Boron substitution for Sb in the Nai50Mn36.5Sb13.5-xBx (x = 0, 0.5, 1.0,1.5) alloys has been studied. The substitution of B reduces the Curie temperaturesof both the martensite and the austenite phases. The saturation magnetization Msdecreases monotonically with increasing B content. The doping of B increases themartensitic transformation temperature effectively.We synthesized a series of Ni₅₀Mn_36.5Sb_13.5-xSi_x (x = 0 - 2.5) alloy. When x =0-2.0, the samples are bcc phase. Since Si has small atomic radius compared withthat of Sb, the lattice parameters decrease with increasing Si content. When x =2.5,a small amount of secondary phase is observed. With increasing Si content,Thesaturation magnetization Ms decreases but the Curie temperature increases. Thedoping of Si decreases the martensitic transformation temperature effectively.Below martensitic transformation temperature, the exchanges bias phenomenon isobserved.We studied the structure stability of Ni₂FeZ (Z=In, Sn, Sb) Heuseler alloys.The ground-state energy of both bcc structure and fcc structure of these alloys were calculated. We compared the different value of the ground-state energy of the threealloys in bcc structure and fcc structure. It may be a theoretical support for thedoping of main group element in Ni₂FeGa alloy.We also studied the main-group-element-rich Heusler alloys CoYAl₂(Y=Cr,Mn, Fe) compounds. The calculation results show that the Cu₂MnAl type structureis more stable than the Hg₂CuTi type in this series of materials. In Hg₂CuTi typestructure, the spin polarization of CoCrAl₂ and CoMnAl₂ has been proved to be100%. Even in CoMnAl₂ with Cu2MnAl type structure, the spin polarization is stillup to 80%. So it can be predicted that main-group-element-rich Heusler alloy isprobably a new material system with high spin polarization.