In Situ Magnetic-Circular-X-Ray-Dichroism Measurements: An Epitaxial Fe Wedge on Cu(100)
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Introduction
MCXD Basics
Circularly polarized x-ray radiation is attracting increasing interest as a tool for the characterization of the electronic, magnetic, and chiral properties of lowdimensional structures. Using circular light (with electric field vector parallel to the orbital plane), a dependence of the measured quantity by changing either the orientation of the light polarization or the magnetization is indicative of the existence of magnetic circular dichroism. 1,2 It can be observed in x-ray absorption spectroscopy (XAS), in which the photon energy is scanned through an absorption threshold exciting a core électron into an unoccupied valence state using circularly polarized light. Synchrotron radiation sources hâve made this technique possible. It can also be observed in photoemission spectroscopy from core and valence levels. Hère we focus on magnetic circular x-ray dichroism (MCXD) in XAS as an element-specific tool to investigate magnetic properties of ultrathin films in situ. The application of magneto-optical sum rules enables the détermination of the orbital and spin magnetic moments per atom from XAS spectra, as well as the easy magnetization direction. 1-4
MCXD-based magnetometry in XAS is extensively used by measuring the L2,3 absorption edges of 3rf-transition metals, where large intensity changes (up to 60%) of the L-edge white Unes are observed upon reversai of either the sample magnetization or the light helicity. The high magnetic contrast obtained, combinée! with the elemental specificity of the technique, allows for the study of very dilute samples such as ultrathin films. We first concentrate on the sélection rules governing MCXD in XAS. In a one-electron picture, the dipole approximation leads to the sélection rule Al = ±1, which détermines the symmetry of the final electronic state, given that of the initial one. For example, p —» s and p —> d transitions are allowed at L edges. Transitions to d final States are favored by a factor of order 50. This allows one often to neglect contributions from s final states as a first approximation. Including light polarization, the sélection rules for ni\ are Am, = 0 (linear light) and Am, = ±1 (circular light). Using circular light, the spin-orbit coupling (SOC) of either the initial or final states allows for a partial transfer of the photon angu-
MRS BULLETIN/JANUARY1999
lar momentum to the spin of the excited core électron, which thus acquires a substantial spin polarization along the light propagation axis. For 2p levels the SOC of the initial state leads to a strong spin polarization. The spin orbit of the final state also plays a rôle, as weak MCXD is also observed at the K edges of the 3d ferromagnets. This spin polarization can be either parallel or antiparallel to the incident photon spin, dépending on the individual level of the 1p doublet and on the light helicity. 1,2 In L-edge XAS, for a transition to take place to final unoccupied d states of the exchange split valence bands, some eV above the Fermi level, the core électron has to
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