The extent to which the d orbitals of a transition metal (TM) atom mix with the conduction electron states of a nonmagnetic host governs fundamental macroscopic properties such as electron transport, magnetic susceptibility, and specific heat. Theoretical predictions as well as experimental studies on bulk TM impurities dissolved into a non-magnetic matrix show that the TM magnetic moments vary, or even disappear, depending on the d-state localisation and host electron density. However, despite extensive study since the work of Friedel and Anderson, the degree of d-state localisation and the magnetic moment of the impurity have remained open questions. Experiments on beamline ID8 have been able to resolve these issues for the first time using a combination of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). These techniques are sensitive to bulk and, in particular, to surface TM impurities with concentration as low as 3 x 1012 atoms cm­2 allowing an unambiguous determination of the impurity valence state, spin (S) and orbital (L) magnetic moment.

The interaction between the localised 3d states of Fe, Co, and Ni and a free-electron gas were studied by separately depositing minute amounts of the respective TM onto thick K films condensed onto a Cu(111) substrate at 10 K. The measurements were performed in magnetic fields of 7 T with the sample held at 10 K. The XAS spectra over the L2,3 edges and resulting XMCD, recorded using left and right circularly-polarised light, are shown in Figure 53. In each case the XAS and XMCD are compared with typical spectra for the bulk case. The lineshapes and relative strengths of the XAS and XMCD features are fingerprints for the atomic d-state configuration. The most striking difference between the impurity and bulk XAS and XMCD spectra is the presence of sharp multiplet features which reveal the TM atomic-like ground states due to strong 3d-electron localisation at the impurity sites. Comparison with calculated atomic spectra [1] (bottom inset in Figure 53b) indicates that a charge transfer, from the free-electron host, increases the d-valence of the TM adatoms by one electron compared to the atomic case, yielding ground states with predominantly Fe d7, Co d8, and Ni d9 character and correspondingly large magnetic moments of 6.6, 5.6, and 3.6 µB, respectively. Further, the XMCD at the L2 edge of Fe and Co has the opposite sign with respect to the bulk spectra, whereas it is zero for Ni. Application of the XMCD sum rules yields atomic-like values for L of approximately 3, 3, and 2µB, respectively, in agreement with the predictions of Hund's rules. The consistency of these results strongly supports the XMCD sum rules in the precise context of their derivation, i.e. for localised atomic states.

Fig. 53: XAS and XMCD spectra for the L2,3 edges recorded with X-ray polarisation vector parallel (black line) and antiparallel (red line) to a 7 T applied magnetic field for (a) Fe, (b) Co, and (c) Ni impurities on K/Cu(111). The top insets show the corresponding bulk spectra. A calculated [1] atomic spectra for a d8 configuration is shown in the bottom inset in (b).

Figure 54shows changes in the XMCD lineshape from Co/K/Cu(111) for progressive cluster formation with increasing Co coverage. During cluster growth, atomic configuration mixing and increasing d-state hybridisation result in broader multiplet features in the XAS and hence also the XMCD. At the L3 edge shown in Figure 54a, the peak broadens, whereas the smaller satellite at 781 eV becomes less well-defined and finally disappears. At the L2 edge, shown in Figure 54b, the XMCD is negative and changes sign with increasing coverage. These changes imply that L is quenched much faster than S.

Fig. 54: XMCD spectra recorded at the (a) L3 and (b) L2 edge for Co on K/Cu(111) showing changes in the lineshape as isolated Co atoms form clusters with increased coverage. The value of L for bulk Co and isolated adatoms is also indicated.

In conclusion, the electronic state and magnetic moment of isolated TM atoms on metal surfaces have been investigated for the first time. TM impurities on a simple metal substrate display localised atomic configurations with atomic orbital magnetic moments. The present results conclusively prove that the giant TM magnetic moments in alkali systems [2] originate from the localisation of the 3d states. Finally, we emphasise that combined XAS and XMCD experiments have an unexplored potential for the study of diluted systems, allowing the simultaneous determination of the electronic and magnetic configuration of the impurity states.

[1] G. van der Laan and B.T. Thole, Phys. Rev. B 43, 13401 (1991).
[2] H. Beckmann and G. Bergmann, Phys. Rev. Lett. 83, 2417 (1999); ibid. 85, 1584 (2000).

Principal Publication and Authors
P. Gambardella (a), S.S. Dhesi (b), S. Gardonio (c), C. Grazioli (c), P. Ohresser (d) and C. Carbone (c), Phys. Rev. Lett. 88, 047202 (2002).
(a) EPF Lausanne (Switzerland)
(b) ESRF
(c) CNR-ISM, Trieste (Italy)
(d) LURE, Paris (France)