Studies On Site Preference In Some Heusler Alloys

This thesis mainly studied the site preference of transition metal elements in Heusler alloys and its influences on the electronic structure and magnetic properties in Heusler alloys X₂CuAl,Fe₂MnGa,Mn₂CoZ,Fe₂RuSi and Co₂RuSi.We suggested other possible factors which can influence the site preference besides the“valence electrons rule”.This can help to understand the atomic ordering in Heusler alloys and to realize more precise material design.The site preference of Cu in Heusler alloys X₂CuAl?X=Sc,Ti,V,Cr,Mn,Fe,Co,Ni?have been investigated by first-principles calculation.A competition of the conventional L2₁ ordering and inverse XA ordering has been observed in X₂CuAl.It is also found that,the site preference of Cu is strongly influenced by the original crystal structure of pure X elements in those alloys.So the crystal structure of pure X element should also be considered together with number of valence electrons when discussing the site preference in Heusler alloys.Here Mn₂CuAl is predicted to be a spin gapless semiconductor?SGS?.Single phase Fe₂MnGa Heusler alloy can be prepared by the melt-spinning technique.The Curie temperature of BCC Fe2MnGa ribbon is about 185 K and the saturation moment at 5 K is 3.68?_B/f.u.,which is much larger than the total moment of about 2?_B calculated in literature as well as in the present work for Fe₂MnGa with highly ordered L2₁ structure.This difference can be explained by the Fe-Mn disorder between A,B sites introduced by melt spinning.The XRD pattern also reveals the possible existence of this kind of disorder.Competition between the highly-ordered XA structure and disordered L2₁B structure in Heusler alloys Mn₂CoZ?Z=Al,Ga,Si,Ge,Sb?has been investigated.It is found that,the relative stability of the two structures strongly depends on the main group element Z.When Z belongs to Al or Ga,the XA structure is stabler,but when Z belongs to Si,Ge or Sb,the L2₁B structure gains stability and is lower in energy.This is related to the different number of valence electrons in main group element Z,which influences the DOS structure near the Fermi level and changes N?E_F?.The site preference,electronic structure and magnetism of Heusler alloy Fe₂RuSi have been investigated theoretically and experimentally.First-principles calculations suggest that even though both Fe and Ru have 8 valence electrons,Ru atoms prefer entering A,C sites in Fe₂RuSi.These recent results suggest that some new possible factors such as atomic radius and covalent hybridization should also be considered.Heusler alloy Co₂RuSi has been synthesized by melt-spinning technology.Co₂RuSi bulk sample after annealing is composed of an HCP Co-rich phase and a BCC Ru-Si phase,but melt-spinning can suppress the precipitation of the HCP phase and produce a single Co₂RuSi Heusler phase.But an endothermic peak is observed at 871 K in the DTA heating curve,corresponding to the decomposition of this metastable phase.In the XRD pattern,an L2₁B-B2 structure has been determined in the melt-spun sample,which means the Ru atom has a strong preference for the?A,C?sites in the lattice of Co₂RuSi,though Ru has fewer valence electrons compared with Co.This can be related to the strong covalent hybridization between Ru-Si and Ru-Co nearest neighbors.