Spin Ordering in LaOFeAs and Its Suppression in Superconductor LaO0.89F0.11FeAs Probed by M\"ossbauer Spectroscopy

TL;DR

This study uses Mössbauer spectroscopy to show that fluorine doping suppresses magnetic order in LaOFeAs, leading to superconductivity in LaO0.89F0.11FeAs by eliminating magnetic and structural transitions present in the parent compound.

Contribution

It provides direct spectroscopic evidence that F-doping suppresses magnetic order and structural transitions, facilitating superconductivity in iron-based layered superconductors.

Findings

No magnetic splitting observed in F-doped superconductor from 4.2 to 298 K.

Parent LaOFeAs shows magnetic transition at ~140 K.

F-doping suppresses magnetic and structural transitions, enabling superconductivity.

Abstract

The 57Fe M\"ossbauer spectroscopy was applied to an iron-based layered superconductor LaO0.89F0.11FeAs with a transition temperature of 26 K and its parent material LaOFeAs. Throughout the temperature range from 4.2 to 298 K, a singlet spectrum with no magnetic splitting was observed as a main component of each M\"ossbauer spectrum of the F-doped superconductor. No additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, the parent LaOFeAs shows a magnetic transition at around 140 K, and this temperature is slightly lower than that of a structural phase transition from tetragonal to orthorhombic phase, which accompanies the resistivity anomaly at around 150 K. The magnetic moment is estimated to be ~0.35 $\mu$B/Fe at 4.2 K in the orthorhombic phase, and the spin disorder remains in the magnetic ordered state even at 4.2 K. The fact that no magnetic transition in LaO0.89F0.11FeAs was observed even at 4.2 K under 7 T implies a strong spin fluctuation above Tc or small magnetic moment in this system. Therefore, the present results show that the F-doping effectively suppresses the magnetic and structural transitions in the parent material and the suppression leads to emergence of superconductivity in this system.