Structure and magnetic properties of Fe-Ni clusters

Using a first-principles molecular-orbital approach, we study the equilibrium geometries and binding energies of small Fe-Ni clusters containing up to four Fe and four Ni atoms. We find that the clusters are ferromagnetic with magnetic moments enhanced relative to the bulk values. For some specific cluster sizes and compositions, different isomers (corresponding to different geometries and clearly different energies) are found to have the same magnetic moment. This behavior is in contrast to the common understanding that magnetism usually depends on the cluster geometry. Calculations have also been carried out by modeling Fen-1Ni (n=2,9,15,27) clusters as fragments of a bcc lattice to determine the preferred site of Ni and its effect on the magnetic properties. The results are used to analyze the recent experimental study of the magnetic properties of FeNi nanoparticles