Thermodynamics of the Heusler alloy Co_2-xMn_1+xSi: a combined density functional theory and cluster expansion study

TL;DR

This study combines density functional theory and cluster expansion methods to analyze the thermodynamics and structural stability of the Co_2-xMn_1+xSi Heusler alloy, predicting conditions for half-metallicity and stability across compositions.

Contribution

It introduces separate cluster expansions for formation energy and spin moment, extending the Slater-Pauling rule to non-stoichiometric compositions, and predicts stability and half-metallicity in Mn-rich alloys.

Findings

High structural order in stoichiometric Co_2MnSi at equilibrium.

Small Mn additions suppress detrimental antisites.

Mn-rich compositions decompose above room temperature.

Abstract

Previous studies indicated that intrinsic point defects play a crucial role for the density of states of ferromagnetic half-metals in the band gap region: At large concentrations, defect-derived bands might close the gap at the Fermi energy in the minority spin channel. In this work, structural disorder in the Co- and Mn-sublattices of the full Heusler alloy Co_2-xMn_1+xSi (-1 < x < 2) is investigated with a cluster expansion approach, parametrized using all-electron density functional theory calculations. By establishing two separate cluster expansions, one for the formation energy and one for the total spin moment, we are in position to determine the stability of different configurations, to predict new (also half-metallic) ground states and to extend the known Slater-Pauling rule for ideally stoichiometric Heusler alloys to non-stoichiometric, Mn-rich compositions. This enables us to identify potentially half-metallic structures in the Mn-rich region. With the help of Monte Carlo simulations based on the cluster expansion, we establish theoretically that Co_2-xMn_1+xSi close to the stoichiometric composition ought to show a high degree of structural order in thermodynamic equilibrium. Hence, samples prepared with the correct stoichiometry should indeed be half-metallic after thermal annealing. Moreover, we predict that adding a small amount of Mn to stoichiometric Co_2MnSi allows to suppress the thermally activated formation of detrimental Co antisites. At Mn-rich compositions (x>1), the ordered ground state structures predicted for zero temperature are found to be thermally unstable and to decompose into Co2MnSi and Mn3Si above room temperature.