Magnetic anisotropy of ultrathin metal films is one of the most attractive subjects in magnetism. When one considers magnetic anisotropy of thin films within the framework of the classical electromagnetic theory, one finds that in-plane magnetization is always more stable than perpendicular magnetization. Perpendicular magnetic anisotropy (PMA) is, however, sometimes observed in real systems and the understanding of the origin of PMA is important from the viewpoints of both fundamental physics and technological applications to new-generation high-density recording media. We have been investigating the microscopic mechanism of PMA that is stabilized by gaseous adsorption on magnetic film surfaces by means of the synchrotron radiation x-ray magnetic circular dichroism (XMCD) technique. A goal of these works is spin engineering by which the magnetization of ultrathin metal films can be controlled artificially.

Recently, we have investigated spin reorientation transitions of ultrathin Co/Pd(111) films induced by adsorption of atoms and molecules by means of Co LIII,II-edge XMCD [D. Matsumura, T. Yokoyama et al. Phys. Rev. B 66 , 024402 (2002)]. Figure 1 shows the Co L_III,II-edge XMCD spectra on clean and CO-adsorbed 4.5 ML Co/Pd(111) at 200 K.

Fig. 1: Co L_III,II-edge circularly polarized (solid and dashed lines) and XMCD (dotted line) spectra of 4.5 ML Co/Pd(111) at 200 K before and after CO adsorption. theta is the angle between surface normal and the x-ray electric field (theta=30 deg. and theta=90 deg. correspond to grazing and normal x-ray incidence, respectively). It is found that the magnetic easy axis rotates from in-plane in clean to perpendicular in CO-adsorbed.

In clean Co/Pd(111) the XMCD signal appears only in the grazing-incidence spectrum, while in CO-adsorbed Co/Pd(111) the normal-incidence spectrum gives a two-times more intense XMCD signal than the grazing-incidence spectrum. These observation directly implies that the magnetization direction varies from in-plane to perpendicular upon CO adsorption. We observed a similar adsorbate-induced spin reorientation transition in the NO case as well, while in O or H adsorption no transitions took place. We have investigated detailed Co-thickness dependence in the case of CO. Figure 2 shows the Co spin magnetic moments of clean and CO-adsorbed Co/Pd(111).

Fig. 2: Co thickness dependence of the Co spin magnetic moments at 200 K. Hatched areas indicate the PMA regions. The critical thickness for the spin reorientation transition is about 3.5 ML for clean Co and about 6.5 ML for CO-adsorbed Co, this indicating that the perpendicular magnetic anisotropy is stabilized upon CO adsorption.

The critical thickness of the spin reorientation transition in CO-adsorbed Co is found to be about 6.5 ML, which is by about 3 ML greater than that of clean Co (3.5 ML), implying the stabilization of PMA by CO adsorption.

The most important information from XMCD is the orbital magnetic moments, which determine the magnetic easy axis in spite of very small contribution to the total magnetic moments. Figure 3 shows the orbital magnetic moments of Co on clean and CO-adsorbed Co/Pd(111).

Fig. 3: Co thickness dependence of the Co orbital magnetic moments at 200 K. After CO adsorption, the perpendicular orbital moment does not vary so much (see the data below 3.5 ML), while the in-plane orbital moment reduces significantly (see the ones above 7 ML).

Below 3.5 ML the surface normal orbital moment is found to be left unchanged, while above 6.5 ML the surface parallel orbital moment is reduced significantly after CO adsorption. We conclude that the observed stabilization of perpendicular magnetic anisotropy due to adsorption is ascribed to quenching of the surface parallel orbital magnetic moment.