Calculation of magnetic moment of inverse-Heusler alloy Fe2CuAl via first-principles-based tight-binding model

Heusler alloys have been known since the 19th century and have fascinated researchers because of their wide range of applications, especially related to their magnetic properties. The magnetic moment of a few of these materials can be predicted by simply counting their valance electrons [this is called the Slater-Pauling (SP) rule]. However, this simple counting rule does not work in many other cases. Inverse-Heusler alloys are a sub-class of Heusler alloys, and in many cases also do not follow the SP rule. Fe2CuAl (FCA) is an inverse-Heusler alloy for which the SP rule predicts a magnetic moment of 2.00 μB. However, experimental results give a magnetic moment of 3.30 μB. Motivated by this discrepancy, we study this material theoretically to gain a microscopic understanding of how the magnetic moment forms. For this purpose, we construct a first-principles-based tight-binding model incorporating an on-site Hubbard repulsion U between each d orbital and a Hund coupling J between different d orbitals of a given atom in the system. We use the Green's function technique and apply the mean-field approximation to solve the model. The results show the magnetic moment of FCA depends on U and J. We find that the magnetic moment given by the SP rule (2.00 μB) corresponds to a plateau in our magnetic moment vs J curves for various values of U, and that the experimental magnetic moment (3.30 μB) corresponds to a point on the curves rising above the plateau.