X-ray absorption spectra in a wide energy range around the 4d-4f excitation threshold of Gd were recorded by total electron yield from in-plane magnetized Gd metal films. Matching the experimental spectra to tabulated absorption data reveals unprecedented short light absorption lengths down to 3 nm. The associated real parts of the refractive index for circularly polarized light propagating parallel or antiparallel to the Gd magnetization, determined through the KramersKronig transformation, correspond to a magneto-optical Faraday rotation of 0.7• per atomic layer. This finding shall allow the study of magnetic structure and magnetization dynamics of lanthanide elements in nanosize systems and dilute alloys.PACS numbers: 78.20.Ls, 78.70.Dm Resonance enhancements of X-ray magnetic scattering cross sections at inner-shell absorption edges 1,2 have been used for years to investigate the magnetic structure of lanthanide 3 and actinide 4 systems in the hard X-ray regime (above 2 keV). The experimental demonstration of large changes in the specularly reflected X-ray intensity at the Fe L 2,3 edge 5 upon magnetization reversal initiated the ongoing search for magneto-optical (MO) effects in the soft X-ray regime, 6,7,8,9,10,11,12 as well as their application to element-specific studies of heteromagnetic systems. 13,14,15,16,17,18 Yet, in analyzing soft X-ray MO signals from thin films and multilayer systems with thicknesses comparable to the X-ray wavelength, previous investigations have shown 5,13,18 that a comparison with model calculations
19of the reflected specular intensity (based on the Fresnel equations) is needed in order to extract a layerresolved sample magnetization profile. Several experimental determinations of soft X-ray MO constants have been reported 8,9,10 for ferromagnetic transition metals in the region of the L 2,3 thresholds, but none so far for the lanthanide elements, despite their wide recognition as, e.g., constituents of exchange-spring magnets 20 and magnetic recording media 21 . Only recently has it been demonstrated that sizeable MO signals are obtained from lanthanide elements in the soft X-ray region at the N 4,5 thresholds.
22Here we show that calibrated N 4,5 absorption spectra from magnetized Gd metal, recorded with circularly polarized (CP) light in the energy interval from 110 to 200 eV, yield an X-ray absorption coefficient up to three times larger than expected. The magnetizationdependent absorption of CP light at the Gd N 4,5 giant resonance maximum, described by the imaginary part of the refraction index, is accompanied by a huge change in light propagation speed upon magnetization reversal, a dispersive effect described by the real part of the refraction index, implying a Faraday rotation (FR) of about 0.7• per atomic layer. Thus, even very small or diluted lanthanide systems are expected to show a measurable effect.The absorption experiments were performed at the high-resolution UE56 undulator beamline 23 of the Berliner Elektronenspeicherring für Synchrotronstrahlung (BES...