Regensburg 2002 – scientific programme

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TT: Tiefe Temperaturen

TT 22: Postersitzung III: Syst. korr. Elektr.: Theorie II (1-15), Metall-Isol.- und Phasenüberg
änge (16-33), SL: Massivmat., Bandl., Pinning, Vortexdyn., Transport, Korngr. (34-43), Niedrigdim. Syst., Magnetotransport (44-63)

TT 22.29: Poster

Thursday, March 14, 2002, 14:00–17:30, A

X-ray absorption study of the spin state transition in La_1−xSr_xCoO₃ single crystals — •Z.-W. Hu¹, M. Haverkort¹, L.H. Tjeng¹, S. Csiszar², M. Kriener¹, C. Zobel¹, H. Kierspel¹, T. Lorentz¹, A. Freimuth¹, H.-J. Lin³, H.-H. Hsieh³, and C.T. Chen³ — ¹II. Physikalisches Institut, Universität zu Köln — ²University of Groningen, The Netherlands — ³SRRC, Hsinchu, Taiwan

We have studied the O 1s and the Co L₂₃ XAS spectra of La_1−xSr_xCoO₃ (x = 0, 0.01, 0.04, 0.125, 0.25) single crystals as a function of temperature from 20 K to 650 K. The low spin state of LaCoO₃ at 20 K is confirmed by a comparison with EuCoO₃. Upon increasing the temperature a clear increase in the intensity of a new pre-edge peak in the O 1s XAS spectrum of LaCoO₃ indicates the increase in the population of the intermediate spin state with a jump at around 90 K. This observation clarifies the discrepancy between previous XAS and magnetic studies concerning the first magnetic transition at 90 K. Above 600 K this new feature becomes dominant. However, the spectrum is different from that of FeO, which has the well known high-spin state of 3d⁶. The significant difference between the Co L₂₃ XAS of LaCoO₃ and the Fe L₂₃ XAS spectra of FeO is also observed. These results exclude the presence of a pure high-spin state at the second magnetic transition temperature around 500 K. In case of Sr doping, the observed spectral changes can be assigned to a combined effect of a spin-state transition and an increased presence of Co(IV) ions. The temperature effects on the spin-state transition extend to the ferromagnetic phase for x = 0.125.