Atomic structure, alloying behavior, and magnetism in small Fe-Pt clusters

Abstract

We report results of the atomic structure, alloying behavior, and magnetism in $\mathrm{F}{\mathrm{e}}_{m}\mathrm{P}{\mathrm{t}}_{n}\phantom{\rule{0.28em}{0ex}}(m+n=2--10)$ clusters using projector augmented wave (PAW) pseudopotential method and spin-polarized generalized gradient approximation (GGA) for the exchange-correlation energy. These results are compared with those obtained by using HCTH exchange-correlation functional and LANL2DZ basis set in the Gaussian program and the overall trends are found to be similar. As in bulk Fe-Pt alloys, clusters with equal composition of Fe and Pt have the largest binding energy and the largest heat of nanoalloy formation for a given number of atoms in the cluster. There are some deviations due to the different symmetries in clusters and in cases where the total number of atoms is odd. The lowest energy isomers tend to maximize bonds between unlike atoms with Fe (Pt) atoms occupying high (low) coordination sites in the core (surface) of the cluster. The binding energy, heat of formation, and the second order difference of the total energy show $\mathrm{F}{\mathrm{e}}_{2}\mathrm{P}{\mathrm{t}}_{2}, \mathrm{F}{\mathrm{e}}_{4}\mathrm{P}{\mathrm{t}}_{4}$, and $\mathrm{F}{\mathrm{e}}_{4}\mathrm{P}{\mathrm{t}}_{6}$ clusters to be the most stable ones among the different clusters we have studied. The magnetic moments on Fe atoms are high in Pt-rich clusters as well as in small Fe-rich clusters and decrease as the aggregation of Fe atoms and the cluster size increases. The maximum value of the magnetic moments on Fe atoms is $\ensuremath{\sim}3.8\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, whereas for Pt atoms it is $1 {\ensuremath{\mu}}_{B}$. These are quite high compared with the values for bulk Fe as well as bulk FePt and $\mathrm{F}{\mathrm{e}}_{3}\mathrm{Pt}$ phases while bulk Pt is nonmagnetic. There is significant charge transfer from those Fe atoms that interact directly with Pt atoms. We discuss the hybridization between the electronic states of Pt and Fe atoms as well as the variation in the magnetic moments on Fe and Pt atoms. Our results provide insight into the understanding of the nanoalloy behavior of Fe-Pt and we hope that this would help to design Fe based nanoalloys and their assemblies with high magnetic moments for strong magnets without rare earths as well as Pt alloy catalysts.