Macroscopic phase separation of superconductivity and ferromagnetism in Sr_0.5Ce_0.5FBiS_2-xSe_x revealed by μSR

The compound Sr_0.5Ce_0.5FBiS₂ belongs to the intensively studied family of layered BiS₂ superconductors. It attracts special attention because superconductivity at T _sc = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T _C = 7.5 K. Recently it was reported that upon replacing S by Se T _C drops and ferromagnetism becomes of an itinerant nature. At the same time T _sc 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 Sr_0.5Ce_0.5FBiS_2-xSe_x 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 _C = 3.4 K and T _C = 3.3 K, respectively. For x = 1.0 (T _sc = 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.