The transmission of synchrotron radiation through magnetized iron at energies above the A>absorption edge shows relative differences for right and left circular polarization of several times 10 ~4. The observed spin dependence of the near-edge photoabsorption is proportional to the difference of the spin densities of the unoccupied bands. In the extended absorption region up to 200 eV above the Fermi level a small spin-dependent absorption is observed and thus is expected to give information on the magnetic neighborhood of the absorbing atom.PACS numbers: 75.50. Bb, 75.10.Lp, 78.70.Dm For a better understanding of the remaining problems of ferromagnetism, especially for a clarification of the controversy between the "localized"-and "itinerant"-electron models, 1,2 further experiments are needed to determine the spin-density distribution within the bands near the Fermi level. Besides studies of spin polarization of the occupied states with spin-resolved photoemission spectroscopy, 3 it is of equal importance to investigate the unoccupied ones, which play an essential role for spin fluctuations and excitations. The first steps to address this interesting problem were taken by spin-polarized inverse photoelectron spectroscopy. 4,5 However, because of the small range of low-energy electrons this method is sensitive only to surface layers, whose magnetic properties may differ from those of the bulk. 6 Furthermore the method used is restricted by the limited energy range in which low-energy photons can be detected efficiently by Geiger-Muller counters. The magneto-optic Kerr effect, which is sensitive to the electron spin polarization of states both below and above the Fermi level, is difficult to interpret. 7 Recently we have shown, 8 ' 9 using 9.17-keV circularly polarized x rays emitted after internal conversion in the decay of excited oriented nuclear states, that the photoabsorption cross section of iron contains at these relatively high energies a small spindependent contribution of (1.7 ±0.4) x 10 ~3. Highenergy storage rings are intense, energy-tunable sources of circularly polarized synchrotron radiation (CPSR) in the kiloelectronvolt energy range observable at welldefined small angles above and below the plane of the electron orbits. Thus it seemed to be feasible to measure the spin dependence of photoabsorption above the K edge of 3d ferromagnets. Similarly one may study spindependent L-and A/-edge absorption in magnetic materials containing 4/ and 5/ elements, respectively. Recently an upper limit of 2xl0~3 was observed for the spin-dependent part of the Gd Z,3-edge absorption in a Fe82Gdi8 alloy. 10 In the course of this work, we have observed 11 a remarkably large spin dependence of the photoabsorption in Gd metal at the L 3 and Li edges of 10 ~2 with opposite sign. Here we present the first observation of spin-dependent absorption in the AT-edge region of iron using CPSR from the storage ring DORIS at the Deutsches Elektron-Synchrotron DESY. Our method is discussed as a new technique to determine spin...

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