Chemical order and magnetic behavior of Fe-dilute fcc Fe-Pd nanoparticles Article uri icon

abstract

  • The chemical order and magnetic behavior of Fe-dilute fcc Fe-Pd nanoparticles are theoretically investigated using many-body potentials derived in the framework of the generalized second moment approximation (SMA) and self-consistent spin-polarized tight-binding electronic structure calculations, respectively. The SMA total energy calculations reveal that surface sites and the core region are not favorable positions for the Fe impurities and that they prefer to accumulate in the subsurface region of the particles, showing a very strong tendency to separate. However, additional contrasting atomic configurations close in energy are also found which could imply the coexistence in real samples of several Fe-Pd nanoparticles with a well-defined composition, but having different chemical orderings. Magnetic properties are first investigated for a single Fe impurity in bulk Pd, allowing an extension of the polarization cloud around the Fe atom much larger than in an ab initio calculation. The results are in good agreement with experiments and serve as a reference to identify surface and size effects in FePd nanoparticles. Nanoparticles containing from 135 to 561 atoms with up to three Fe substitutional impurities are then investigated, as well as more concentrated (10%25 Fe content) shell structures. The extension and magnetic structure of the Fe-induced polarization cloud is studied in detail as a function of the size, surface termination, and the precise location and number of the iron impurities in the particles. The local electronic structure at the Pd sites located at the outermost atomic shell is considerably perturbed by the subsurface position of the Fe atoms and could modify the catalytic properties of palladium nanoparticles. Finally, we show that the value of the orbital-to-spin ratio in our Fe-Pd clusters is very sensitive to the changes in the internal position of the Fe impurities, a result that suggests that x-ray magnetic circular dichroism experiments can be very useful in order to reveal precise features of the chemical order in these magnetic nanoparticles. © 2006 The American Physical Society.

publication date

  • 2006-01-01