Oxidation-induced spin reorientation in Co adatoms and CoPd dimers on Ni/Cu(100)

Ultrasmall magnetic clusters and adatoms are of strong current interest because of their possible use in future technological applications. Here, we demonstrate that the magnetic coupling between the adsorbates and the substrate can be significantly changed through oxidation. The magnetic properties of Co adatoms and CoPd dimers deposited on a remanently magnetized Ni/Cu(100) substrate have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy at the Co ${\mathrm{L}}_{2,3}$ edges. Using spectral differences, pure and oxidized components are distinguished, and their respective magnetic moments are determined. The Co adatoms and the CoPd dimers are coupled ferromagnetically to the substrate, while their oxides, Co-O and CoPd-O, are coupled antiferromagnetically to the substrate. Along with the spin reorientation from the pure to the oxidized state, the magnetic moment of the adatom is highly reduced from Co to Co-O. In contrast, the magnetic moment of the dimer is of similar order for CoPd and CoPd-O.

Figure 1

(a) Mass spectrum of Co and CoPd alloy cluster ions produced from the CoPd target. (b) Sketch for the soft landing procedure used for the clusters. (c) (From top to bottom) X-ray absorption and magnetic dichroism spectroscopy for the Ni film (top axis), the Co adatoms, and the CoPd dimers (bottom axis), respectively. The same negative signal at the Ni and Co ${\mathrm{L}}_3$ edge indicates parallel spin moments of the Co adatom, the CoPd dimer, and the Ni film.

Figure 2

(a) L-edge XAS and (b) XMCD spectra of Co, Co-O, CoPd and ${\mathrm{CoPd}}_{\mathrm{ox}.}$ adatoms (from bottom to top), respectively. Along with the appearance of the multiplet structures in the XAS of Co-O and ${\mathrm{CoPd}}_{\mathrm{ox}.}$, the XMCD signal is reduced. (c) and (d) Comparison of the XAS of Co-O and ${\mathrm{CoPd}}_{\mathrm{ox}.}$, using the simulated ${\mathrm{Co}}^{2+}$ spectra from CTM4XAS only (Co-O) and, calculated by linear combination, together with the experimental CoPd XAS spectrum, respectively.

Figure 3

The XAS (a) and XMCD (b) spectra of ${\mathrm{CoPd}}_{\mathrm{ox}.}$ together with the components from CoPd (60%) and CoPd-O (40%). Note that the labeling is changed from ${\mathrm{CoPd}}_{\mathrm{ox}.}$ to $\mathrm{CoPd}\text{−}{\mathrm{O}}_{0.4}$ due to the latter contribution. (c) Sketch for the parallel and antiparallel magnetization alignment of CoPd and CoPd-O in the ${\mathrm{CoPd}}_{\mathrm{ox}.}$ sample. (d) Normalized XMCD for CoPd and CoPd-O adatoms.

Figure 4

${\mathrm{L}}_{2,3}$ edge XAS (a) and XMCD (b) of different CoPd preparations with changing oxidation rate. The circles resemble experimental data while the solid lines are calculated. From the latter, the oxidation rate of the samples CoPd-${\mathrm{O}}_{0.05}$, CoPd-${\mathrm{O}}_{0.25}$, and CoPd-${\mathrm{O}}_{0.40}$ are derived to be 5%, 25%, and 40%, respectively. At the bottom, CoPd-O XAS (simulated by LFM) and XMCD (calculated by linear combination) spectra are shown.

Figure 5

The effective magnetic spin moment ${\mathrm{m}}_{\mathrm{S}}^{\mathrm{eff}}$ for the Co and CoPd dimer is plotted in dependence to the oxidation rate. The negative values of Co-O and CoPd-O indicate an antiparallel coupling to the Ni substrate. The moment of CoPd-O is obtained by a linear extrapolation, as discussed in the text.