Magnetic properties of small size-selected Co and CoPt clusters on Ni

Size and stoichiometry-dependent magnetic properties of deposited Co and CoPt alloy clusters on a Ni/Cu(100) substrate have been investigated by use of x-ray magnetic circular dichroism spectroscopy. Using sum rules, spin and orbital moments were extracted from the spectra. Spin and orbital moments show a strong size-dependent effect with a tendency for an increase with cluster size, which is in contrast to observations for Co clusters on a Pt(111) surface. Addition of a Pt atom to a Co dimer leads to a doubling of the orbital moment. CoPt clusters show a strong increase in chemical reactivity towards oxidation. The resulting Co${}_n$PtO${}_x$ clusters show a ratio of orbital to spin moment comparable to the pure clusters, however, with a strong suppression of the absolute orbital and spin moments.

Figure 1

Mass spectrum of a Co${}_{25}$Pt${}_{75}$ sputter target (open graph: signal $\times$10). The decrease of the pure Co and pure Pt clusters follow a normal exponential decay[14] of cluster-ion-current intensity, while mixed clusters containing Pt and Co are more stable than pure Pt clusters.

Figure 2

XMCD spectra of different Co${}_n$Pt${}_m$ clusters normalized to the L${}_2$ signal deposited on an out-of-plane magnetized Ni/Cu(100) substrate. For a better comparison of the spectra the energy scale is given relative to the maximum XMCD signal at the Co L${}_3$ edge.

Figure 3

L${}_3$ x-ray absorption spectra of deposited Co${}_1$, Co${}_1$Pt${}_1$ clusters and CoO nanoparticles.[17] The energy scale is given relative to the Co L${}_3$ edge.

Figure 4

Co${}_{1,2,3}$ and Co${}_2$Pt${}_1$ orbital ($\blacksquare$) and spin ($•$) magnetic moments per $3d$ hole and their ratio ($▴$) compared to the bulk values of cobalt (dashed line)[26] and CoPt${}_3$ (dotted line).[23]