Scanning SQUID Microscopy on Pulsed-Laser-Deposited FeSe Superconducting Films

FeSe thin films on SrTiO₃ (STO) have attracted interest due to superconducting transition temperatures (T_c) exceeding bulk values, likely driven by interface-enhanced pairing. However, the spatial homogeneity of the superconductivity remains poorly characterized. We use scanning SQUID magnetometry and susceptometry to probe spatially resolved magnetic signatures in pulsed-laser-deposited FeSe films on STO and CaF₂.

Susceptibility scans reveal inhomogeneous Meissner screening. Fitting the data yields local estimates of the Pearl length and T_c. Vortex imaging shows isotropic Pearl vortex profiles, placing an upper bound on the in-plane anisotropy. A discrepancy between the transport T_c and onset of Meissner screening suggests the onset of local superconductivity before global superconductivity.

Weak ferromagnetic domains are absent on bare substrates, indicating that they are associated with the FeSe layer. These persist above 90 K. Susceptibility data suggest spin-glass-like freezing of localized moments, qualitatively resembling prior muSR on Fe(Se,S)/LaAlO₃ thin films, which reported nearly static and spatially inhomogeneous magnetic order. These findings demonstrate the utility of scanning SQUID microscopy for revealing spatial inhomogeneities in superconducting and magnetic properties of FeSe thin films.