Bimetallic Fe−Ni Cluster Alloys: Stability of Core(Fe)−Shell(Ni) Arrays and Their Role Played in the Structure and Magnetic Behavior

Abstract

We present density functional theory as well as tight-binding calculations to analyze the stability and magnetic properties of small fcc and polyicosahedral Fe−Ni cluster alloys having different compositions and chemical orderings. In agreement with the gas-phase experiments of Parks et al. (Chem. Phys. 2000, 262, 151), we obtain in all our considered alloys that iron atoms prefer to accumulate in the core region of the structures leading to the formation of highly stable core(Fe)−shell(Ni) arrays. We found that the low energy atomic configurations of our fcc Fe−Ni clusters strongly depend on the chemical order within the structures. Interestingly, fcc particles with Fe-rich core regions are characterized by unusual and considerably expanded Fe−Fe bond lengths (∼2.7 Å), a result that induces a highly nonuniform relaxation profile in which both sizable expansions and contractions of the interatomic distance between neighboring atoms can be obtained. In contrast, disordered alloys and core(Ni)−shell(Fe) arrays are characterized by Fe−Ni bond lengths of ∼2.5 Å, being of the order of the arithmetic weighted average of the Fe−Fe and Ni−Ni pairs. A common feature of our results is that all our considered Fe−Ni clusters exhibit high-spin ground states (larger than the corresponding bulk alloy values for the same composition); however, we have found that iron clustering within the particles, the presence of chemisorbed hydrogens on the surface, as well as the existence of antiferromagnetic order in the iron-rich regions could lead to a sizable quenching of the average magnetization in the nanoalloys, which is in good agreement with the experimental data of Li et al. (J. Magn. Magn. Mater. 1997, 170, 339). Finally, we show that the value of the orbital-to-spin ratio in both fcc and polyicosahedral Fe−Ni clusters is very sensitive to the internal location of the Fe impurities, a result that suggests that X-ray magnetic circular dichroism experiments can be very useful to reveal precise features of the chemical order in magnetic cluster alloys.