Resonant X-ray magnetic scattering (RXMS) is based on the enhancement of the - otherwise weak - magnetic scattering cross section when the photon energy is tuned to an absorption edge. The resonant process is interpreted in terms of an electric multipolar transition between a core level and the vacant states of a polarised electronic shell. At the L2,3 edges of rare earths, the transitions can be dipolar (2p-5d-2p), and the resonance will carry information on the polarisation of the 5d band, or quadrupolar (2p-4f-2p), reflecting the polarisation of the 4f shell. Our detailed spectroscopic study of the resonance at the L3 edge of samarium in a single crystal epitaxial film shows how the exploitation of the resonant process can lead to a deeper understanding of long range magnetic order in samarium. Using RXMS, we could measure quantities representative of the 4f and 5d polarisation independently, which is an important complement to neutron scattering.

The experiments were carried out at beamline BM28 (XMaS, UK CRG) on a samarium 5000 Å epitaxial film, grown by Molecular Beam Epitaxy (MBE) in the Laboratoire de Physique des Matériaux, University of Nancy, France. The energy dependence of the magnetic intensities shows three resonances, one below and two above the absorption edge (Figure 79b) [1]. The study of the dependence of the resonant intensities with the wave vector transfer shows that the resonance observed below the L3 edge is quadrupolar in origin, probing 4f magnetism, and can be fully interpreted using atomic considerations. The energy dependence of the dipolar resonance at and above the L3 absorption edge shows two maxima and can be interpreted in the framework of a model density of 5d states D(E) (Figure 79c). The same D(E) accounts for both the white line in the fluorescence (solid line in Figure 79a) and for the energy line shape of the resonance (Figure 79b). A similar experiment has been conducted on a bulk samarium single crystal [2], also on BM28. The energy dependence also shows a single quadrupolar and a double dipolar resonances.

The temperature dependence of the magnetic intensity has been studied at two energies, 6.712 keV and 6.719 keV, chosen as representative of the polarisation of the 4f and 5d electrons respectively (Figure 80). The dipolar and quadrupolar resonances follow the same temperature dependence, which implies that the 4f and 5d electrons are polarised in the same ratio, even very close to TN, substantiating the RKKY model for long range magnetic order in rare earths.

[1] A. Stunault, C. Vettier, N. Bernhoeft, F. de Bergevin, C. Dufour, K. Dumesnil, SPIE Proceedings, 3773, 295-303 (1999).
[2] A.D.F. Herring, W.G. Stirling, N. Burton, M.M.R. Costa, A. Stunault, to be published.

A. Stunault (a,b), N. Bernhoeft (c), K. Dumesnil(d), C. Dufour (d), C. Vettier (e).

(a) ESRF
(b) Department of Physics, University of Liverpool (UK)
(c) CENG, Grenoble (France)
(d) Laboratoire de Physique des Matériaux, Université Henri Poincaré-Nancy 1, Vandoeuvre les Nancy, (France)
(e) ILL, Grenoble (France)