Atomic Disorder Effects On Half-Metallcity Of Mn2FeSb And Mn2CoAl Compounds:a First-Principles Study

Heusler compounds are a huge family of ternary and quaternary compounds, which have more than 1000 known members, and most of them crystallize in cubic structures. Since the discovery of the half-metallicity in the half-Heusler NiMnSb compounds, Heusler compounds attracted great attention because they have high Curie temperatures, high spin polarizations, and structural similarities with zinc-blende semiconductors. Half-metallic materials present metallic behavior for one of the spin-direction, while they have an energy gap around the Fermi level in the other spin direction, leading to 100% spin-polarization at the Fermi level. The measured spin polarizations for some half-metals are much lower than the expected value of 100%, which can be ascribed to the atomic disorder or the surface effect.In this paper, we investigate the effects of atomic disorder on the electronic structures, magnetism, and half-metallicity of full-Heusler Mn2CoAl and Mn2FeSb compounds with Hg2CuTi structure by using the full-potential linearized-augmented planewave (FLAPW) method within the generalized gradient approximation (GGA).We have considered five types of atomic disorders, i.e., Mn(A)-Fe(Co), Mn(A)-Sb(Al), Mn(B)-Fe(Co), Mn(B)-Sb(Al) and Fe(Co)-Sb(Al)atomic disorders. To describe a low and high level of disorder,we have adopted two unit cells:one is a cubic Hg2CuTi structure containing 16 atoms and the other contains 32 atoms obtained by doubling the primitive unit cell of Hg2CuTi structure.The calculated results are as follows:(1) For the Mn2FeSb compound, only the M(B)-Sb atomic disorder maintains the half-metallicity, regardless of the disorder level. On the other hand, the Mn(A)-Fe, Mn(A)-Sb, Mn(B)-Fe, and Fe-Sb atomic disorders destroy the half-metallicity due to additional states appear in the minority band-gap at the Fermi level. Total energy calculations show that the Mn(A)-Fe atomic disorder has the lowest total energy, implying the atomic disorder is more likely to occur than the others.(2) For the Mn2CoAl compound, the Mn(A)-Co and Mn(B)-Co atomic disorders maintain the half-metallicity, while the Co-Al atomic disorder destroys the half-metallicity, regardless of the disorder level. The half-metallicity retains only at the low disorder level of the Mn(A)-Al and Mn(B)-Al atomic disorders. The Mn(A)-Co atomic disorder has the lowest total energy and is more likely to occur among the atomic disorders. In addition, the spin gapless semiconducting character of Mn2CoAlcompound was destroyed by atomic disorders.