Supercurrent-controlled spontaneous ferromagnetism of magnetic impurities in a spin-orbit-coupled superconductor

Conventional superconductors disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. Consequently, ferromagnetic coupling between localized moments through the charge carriers is absent in s-wave superconductors. However, Rashba-type spin-orbit coupling (SOC) can profoundly alter the interaction between the charge carriers and localized spins. Here, we report long-range ferromagnetic order among the impurity moments on the surface of an s-wave SOC superconductor, Fe(Se, Te). We employ scanning superconducting quantum interference device microscopy and find suppressed vortex states but spontaneous magnetic domains in the superconducting state. Homogeneous remanent flux patterns are induced by applying supercurrents through the sample. The patterns are consistent with anomalous edge and bulk supercurrents generated by long-range in-plane magnetization related to the magneto-electric effect intrinsic to a Rashba superconductor. The anomalous supercurrents occur above a bias current threshold and follow hysteresis loops reminiscent of those of a ferromagnet. Surprisingly, the magnetic switching (super)current density is three orders of magnitude less than that in a ferromagnetic metal with SOC. These observations suggest an emergent ferromagnetic impurity state in a Rashba superconductor, which can be controlled by applying supercurrents. In conventional s-wave superconductors, ferromagnetism is strongly suppressed, due to the singlet pairing of the electrons. Here, Xiang, He, Lin, and coauthors find a long-range ferromagnetic order among impurity sites in the spin-orbit coupled Fe(Se,Te) using scanning superconducting quantum interference device microscopy.