Electronic structure of sub-stoichiometric iron aluminide clusters

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 theor y and the generalized gradient approximation for the exchange-correlation p otential. The preferred location of Al atoms as well as the atomic relaxati ons following Al substitution are determined by minimizing the total energy of the cluster subject to certain symmetry constraints. The electronic ene rgy levels near the Fermi energy are found to be dominated by Fe 3d orbital s for x < 0.33. For higher aluminium concentrations, the density of states for the highest occupied molecular orbitals (HOMOs) and lowest unoccupied m olecular orbitals (LUMOs) are a strong admixture of Fe 3d and Al 3p orbital s. The filling of the minority spin states of Fe 3d followed by the shiftin g of the Fermi energy towards Al 3p with successive doping of Al is consist ent with the observed anomaly in the electrical resistivity of iron alumini des. This change in the electronic structure is also found to have a signif icant impact on the magnetic properties of these systems. While the magneti c moment at the individual Fe sites decreases from 3 mu (B) to 2 mu (B) wit h increasing Al concentration, the net magnetization undergoes substantial reduction not only because of decreasing Fe content but also because of ant i-ferromagnetic coupling between Fe and Al sites. The ability of a finite s ize cluster to model bulk behaviour is examined.