Using a first-principles molecular-orbital approach, we study the equilibri
um 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 wit
h magnetic moments enhanced relative to the bulk values. For some specific
cluster sizes and compositions, different isomers (corresponding to differe
nt geometries and clearly different energies) are found to have the same ma
gnetic moment. This behavior is in contrast to the common understanding tha
t 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 t
he magnetic properties. The results are used to analyze the recent experime
ntal study of the magnetic properties of FeNi nanoparticles.