Correlation of shape and magnetic anisotropy of supported mass-filtered Fe and FeCo alloy nanoparticles on W(110)

Mass-filtered Fe and FeCo nanoparticles are deposited under soft-landing and ultra-high vacuum conditions on the bare W(110) surface. The structure and the stoichiometry of FeCo alloy nanoparticles are determined by ex situ high resolution transmission electron microscopy (HRTEM) and energy-dispersive x-ray spectroscopy (EDX) on single nanoparticles, respectively, proving a crystalline structure. In situ scanning tunneling microscopy (STM) shows that the clusters are irregularly distributed on the W(110) surface, that no fragmentation occurs, that steps do not act as preferential adsorption sites, and yields strong evidence for a partial flattening of the particles upon deposition. The magnetic anisotropy energy (MAE) of mass-filtered pure Fe nanoparticles is investigated by means of x-ray magnetic circular dichroism (XMCD) at the iron L3 edge. The magnetization loops reveal an uniaxial magnetic anisotropy with the magnetic hard axis being perpendicular to the surface. The experimentally determined MAE is compared to calculated shape and interface anisotropy contributions of partially flattened nanoparticles according to the STM observations on the FeCo clusters.