Electronic structure of sub-stoichiometric iron aluminide clusters

Abstract

The electronic structure of FenAlm (n + m = 15) clusters mimicking Fe1−xAlx alloys in the 0 < x < 0.5 composition range is investigated systematically by modelling the system with a 15-atom cluster having a body-centred cubic structure. The calculations are carried out using density-functional theory and the generalized gradient approximation for the exchange-correlation potential. The preferred location of Al atoms as well as the atomic relaxations following Al substitution are determined by minimizing the total energy of the cluster subject to certain symmetry constraints. The electronic energy levels near the Fermi energy are found to be dominated by Fe 3d orbitals for x < 0.33. For higher aluminium concentrations, the density of states for the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are a strong admixture of Fe 3d and Al 3p orbitals. The filling of the minority spin states of Fe 3d followed by the shifting of the Fermi energy towards Al 3p with successive doping of Al is consistent with the observed anomaly in the electrical resistivity of iron aluminides. This change in the electronic structure is also found to have a significant impact on the magnetic properties of these systems. While the magnetic moment at the individual Fe sites decreases from 3 μB to 2 μB with increasing Al concentration, the net magnetization undergoes substantial reduction not only because of decreasing Fe content but also because of anti-ferromagnetic coupling between Fe and Al sites. The ability of a finite size cluster to model bulk behaviour is examined.