Competition of XA and L21B ordering in Heusler alloys Mn2CoZ (Z = Al, Ga, Si, Ge and Sb) and its influence on electronic structure

Abstract Competition between the highly-ordered XA structure and disordered L2 1 B structure in Heusler alloys Mn 2 CoZ (Z = Al, Ga, Si, Ge, Sb) has been investigated. 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 L2 1 B 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 energy difference ΔE between the XA and L2 1 B structures may be used to estimate the tendency to form L2 1 B structure in different Heusler alloys qualitatively. A large negative ΔE is preferable to retard the A-C site disorder and retain the highly-ordered XA structure. That is just the case in Mn 2 CoAl. A robust half-metallicity is observed in Mn 2 CoSi, Mn 2 CoGe and Mn 2 CoSb, they are always half-metallic under either XA or L2 1 B structure. But in Mn 2 CoAl and Mn 2 CoGa, their spin gapless semiconducting character will be destroyed and replaced by a half-metallic state if L2 1 B disorder occurs. Finally, these results suggest that the L2 1 B structure should be considered together with XA structure when discussing the electronic structure of “inverse” Heusler alloys.