Macroscopic phase separation of superconductivity and ferromagnetism in Sr0.5Ce0.5FBiS2-xSex revealed by muSR

Abstract

The compound Sr${}_{0.5}$Ce${}_{0.5}$FBiS${}_2$ belongs to the intensively studied family of layered BiS${}_2$ 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 (Thakur et al., Sci. Reports 6, 37527 (2016)). 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 ($\mu$SR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr${}_{0.5}$Ce${}_{0.5}$FBiS${}_{2-x}$Se${}_x$ 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 $\sim 8\%$ and $\sim 30\%$ 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 the remaining sample volume ($\sim 70\%$), 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.

9 pages (includes 4 figures) + supplementary information file