First principles calculations of structural, magnetic and electronic properties of Co2TiZ (ZÂ = Si and Sn) Heusler alloys using LSDA+U method: Effect of U

Using first principles LSDA+U calculation, a systematic investigation of the role of the Hubbard potential U on the structural, magnetic and electronic properties of Co2TiSi and Co2TiSn Heusler alloys was conducted. The structural, magnetic and electronic properties of Co2TiSi and Co2TiSn were calculated at different values of Hubbard potentials for Co (UCo) and Ti (UTi) atoms. The calculated lattice parameters are found to be slightly underestimated when compared with the experimental values with insignificant dependence on both UCo and UTi. While non integer magnetic moments and metallic behaviors were predicted by LSDA, LSDA+U predicted half metallic behaviors and magnetic moments of 2μB, in agreement with Slater-Pauling rule, at relatively low values of UCo and UTi. A strong dependence of the band gap (Eg) on UCo and UTi was found in the two systems. The Eg(U) dependence is characterized by two distinct regions: first region shows increasing trend while second region shows saturation with the increase in the value of UCo. In the first region, the band gap is controlled by Co d-t2g and Co d-eg suborbitals only, and UTi plays no role in agreement with Glanakis et al. scheme. However, in the second region, the band gap is defined by the Co d-t2g and Ti d-eg suborbitals. The band gaps of the alloys were also calculated using the modified Becke-Johnson (mBJ) method predicting values within the second regions. The role of UCo and UTi in determining the Eg of the two systems was discussed.