Spin Ordering in LaFeAsO and Its Suppression in Superconductor LaFeAsO_<0.89>F_<0.11> Probed by Mossbauer Spectroscopy(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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^<57>Fe Mossbauer spectroscopy was applied to an iron-based layered superconductor LaFeAsO_<0.89>F_<0.11> with a transition temperature of 26K and to its parent material LaFeAsO. Throughout the temperature range from 4.2 to 298K, a singlet pattern with no magnetic splitting was observed in the Mossbauer spectrum of the F-doped superconductor. Furthermore, no additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7T. On the other hand, magnetically split spectra were observed in the parent LaFeAsO below 140K, and this temperature is slightly lower than that of a structural phase transition from tetragonal to orthorhombic phase, which accompanies the electrical resistivity anomaly at around 150K. The magnetic moment is estimated to be 〜0.35μ_B/Fe from the internal magnetic field of 5.3T at 4.2K in the orthorhombic phase, and the spin disorder appears to remain in the magnetically ordered state even at 4.2K. The lack of a magnetic transition in LaFeAsO_<0.89>F_<0.11> down to 4.2K suggests that this system exhibits a paramagnetic state or that the magnetic moment is small. The present results show that F doping effectively suppresses the magnetic and structural transitions in the parent material, leading to the emergence of superconductivity in the F-doped system.
社団法人日本物理学会の論文
2008-10-15