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

TL;DR

This study uses muon spin rotation to show that in Sr$_{0.5}$Ce$_{0.5}$FBiS$_{2-x}$Se$_x$, superconductivity and ferromagnetism do not coexist microscopically but are separated into different regions, with implications for understanding their interplay.

Contribution

The paper provides direct experimental evidence that superconductivity and ferromagnetism are macroscopically phase separated in Sr$_{0.5}$Ce$_{0.5}$FBiS$_{2-x}$Se$_x$, challenging previous claims of microscopic coexistence.

Findings

Ferromagnetic and superconducting phases occupy different sample volumes.

Ferromagnetic volume fraction is about 8% for x=0.5 and 30% for x=1.0.

Superconductivity occupies approximately 70% of the sample volume for x=1.0.

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.