Evidence for Local Moment by Electron Spin Resonance Study on Polycrystalline LaFeAsO$_{1-x}$F$_x$ (x=0 and 0.13)

TL;DR

This study uses electron spin resonance to demonstrate the presence of local magnetic moments and magnetic fluctuations in LaFeAsO$_{1-x}$F$_x$ superconductors, suggesting a magnetic fluctuation mechanism for superconductivity.

Contribution

It provides experimental evidence for local moments and magnetic frustration in LaFeAsO$_{1-x}$F$_x$, highlighting their potential role in superconductivity.

Findings

ESR signals indicate temperature-dependent local moments.

Strong magnetic frustration observed in the system.

Results support magnetic fluctuation as a mechanism for superconductivity.

Abstract

The temperature dependence of electron spin resonance (ESR) was studied in the oxypnictide superconductors LaFeAsO$_{1-x}$F$_x$ (x = 0.0 and 0.13). In the samples, the ESR signal indicates that the g factor and peak-to-peak linewidth strongly depend on temperature, especially at low temperatures. It indicates a strong coupling picture with existence of local moment. The dependence mentioned above gradually attenuates, and tends to saturation around room-temperature. This behavior could be ascribed to "bottleneck" effect due to coupling of local moment and itinerant electron. In addition, a Curie-Weiss like behavior is also observed in temperature dependent integral intensity for the two samples. Our results strongly support the existence of local moments in these materials while its origin is still unclear. The results also indicate strong magnetic frustration in this system, and magnetic fluctuation mechanism for superconductivity is suggested.