15th International Conference on Muon Spin Rotation, Relaxation and Resonance

The Fe-based superconductors have been extensively investigated in view of the intimate interplay of the magnetic and superconducting phases developing therein. Here, we show an anomalous behaviour of the mixed phase in the family of superconducting compounds LaFeAsO$_{1-x}$F$_{x}$ as detected by transverse-field muon spin rotation. This technique is the best tool to probe both the flux line lattice distribution and any additional source of magnetism through both the muon depolarization rate and the local magnetic field at the muon site $B_{\mu}$. Although the typical shielding behaviour is expected to saturate to a constant value well below the superconducting transition $T_{c}$, our results for LaFeAsO$_{1-x}$F$_{x}$ show that $B_{\mu}$ is enhanced in the low-temperature regime (see figure). We discuss an extensive investigation of this phenomenon as a function of F doping, external magnetic field and temperature alongside different interpretations for the observed phenomenology, including a spontaneous magnetic re-entrance and a possible change of the flux line lattice distribution [1].

[1] Manuscript in preparation (2022).