Effect of vacancies on the electronic and magnetic properties of Heusler compound Mn2CoAl

Investigation of the effect of vacancies occurring within the crystal structure of Heusler compound Mn2CoAl, reveal the emergence of half-metallicity or a metallic electronic structure, depending on the site where the vacancy occurs. The Density Functional Theory (DFT) method, as implemented in the Vienna Ab Initio Simulation package (VASP) has been used. The site specific vacancies reveal that for a Mn vacancy occurring at the tetrahedral site and for a Co vacancy, an increased intersection of states with the Fermi level occurs, with the emergence of half-metallicity and vacancy-induced states in the minority spin channel gap respectively. The resultant spin polarizations are 68% and 37% for the systems with lowest Mn and Co vacancy densities considered in this study and the magnetic moments are 2.00 and 2.03 μB/formula unit respectively. These values are close to the 2 μB/formula unit predicted for Mn2CoAl by the Slater-Pauling rule for half-metallic systems. Simultaneous tetrahedral Mn and Co vacancies induce similar changes in the magnetic moments and density of states but exhibit lower spin polarizations. An octahedral Mn void as well as simultaneous octahedral Mn and Co vacancies result in a metallic electronic structure. In both of these cases, there is a marked deviation from the Slater-Pauling rule for half metals, with magnetic moments of 1.35 and 1.17 μB/formula unit respectively, for the lowest vacancy densities considered in this study. The Mn magnetic moments couple ferrimagnetically and the magnetic order is preserved in all cases, except in the case of the simultaneous Co and Mn vacancies at the octahedral site.