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TT: Tiefe Temperaturen

TT 20: Metall-Isolator-Überg
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TT 20.4: Vortrag

Mittwoch, 10. März 2004, 15:15–15:30, H18

X-ray magnetic and natural circular dichroism at the vicinity of the Co low-spin to high-spin transition in REBaCo₂O_5.5 (RE= Dy; Gd) system — •S. Gold¹, E. Goering¹, P. Lemmens², A. Podlesnyak³, G. Bychkov⁴, J. Deisenhofer⁵, A. Loidl⁵ und G. Schütz¹ — ¹MPI für Metallforschung, Stuttgart — ²MPI für Festkörperforschung, Stuttgart — ³PSI, ETHZ&PSI, Villingen — ⁴Inst. of Solid State & Semicond. Phys., Nat. Acad. Sci. Minsk — ⁵EP5, Universität Augsburg

The magnetic perovskite REBaCo₂O_5.5 (RE= Dy; Gd) system shows a rich magnetic and electronic phase diagram, with a MI transition at higher temperatures and ferromagnetic ordering just below room temperature. With a further reduction of temperature the ferromagnetic magnetization is strongly reduced. This has been discussed in terms of a Co spin state transition and/or antiferromagnetism. A double peak like structure could be observed in the Co L_2,3 XMCD. We interpret this double peak by spectroscopic distinguishable octahedral and pyramidal Co sites. The Co L_2,3 XMCD has been separated from the overlapping Ba M_4,5 spectra by a recently developed XMCD analysis tool called moment analysis. This analysis exhibits an unusual large orbital/spin-moment ratio. In addition, we could observe an extreme large X-ray natural circular dichroism (XNCD) signal, which is about two orders of magnitude larger compared to other published results and about 20% of the corresponding white line intensity. This suggests a large non centrosymmetric charge distribution at the Co site. Nevertheless, the orgin of this extreme large XNCD effect is not clear up to now.