Macroscopic phase separation of superconductivity and ferromagnetism in SrCeFBiS Se revealed by μSR.

The compound SrCeFBiS belongs to the intensively studied family of layered BiS superconductors. It attracts special attention because superconductivity at T  = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T  = 7.5 K. Recently it was reported that upon replacing S by Se T drops and ferromagnetism becomes of an itinerant nature. At the same time T increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study (μSR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in SrCeFBiS Se with x = 0.5 and 1.0. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For x = 0.5 and x = 1.0 we find a ferromagnetic volume fraction of ~8 % and ~30 % at T = 0.25 K, well below T  = 3.4 K and T  = 3.3 K, respectively. For x = 1.0 (T  = 2.9 K) the superconducting phase occupies most (~64 %) of the remaining sample volume, as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated.