Magnetic moment of Fe in oxide-free FePt nanoparticles

Abstract

We present results of x-ray absorption spectroscopy and x-ray magnetic circular dichroism investigations at the ${L}_{3,2}$ edges of Fe in FePt nanoparticles. Plasma treated, oxide-free FePt nanoparticles with a high degree of $L{1}_{0}$ ordering and a mean diameter of about $6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were produced by gas phase condensation followed by in situ flight annealing. The presence of the $L{1}_{0}$ phase is observed by high resolution transmission electron microscopy. Compared to chemically disordered A1 FePt particles of similar size and composition, we find a fourfold enhanced orbital magnetic moment ${\ensuremath{\mu}}_{L}=0.19$ $(\ifmmode\pm\else\textpm\fi{}0.04){\ensuremath{\mu}}_{B}$, a nearly unchanged effective spin magnetic moment ${\ensuremath{\mu}}_{S}^{\mathrm{eff}}=2.21$ $(\ifmmode\pm\else\textpm\fi{}0.24){\ensuremath{\mu}}_{B}$ and a 2% increase in the white-line ratio of the ${L}_{3}$ and ${L}_{2}$ Fe edges. Nevertheless, the room temperature coercive field amounts to ${\ensuremath{\mu}}_{0}{H}_{C}=38(\ifmmode\pm\else\textpm\fi{}7)\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ only. These results are discussed in relation to possible effects of chemical order and surface anisotropy.