Superconductivity-driven ferromagnetism and spin manipulation using vortices in the magnetic superconductor EuRbFe4As4

The coexistence of superconductivity and magnetism has been a long-standing issue in the field of superconductivity due to the antagonistic nature of these two ordered states. Magnetic superconductors are specific materials exhibiting the two antagonistic phenomena whose mutual interaction induces various emergent phenomena, such as the re-entrant superconducting transition associated with the suppression of superconductivity around the magnetic transition temperature (Tm), highlighting the impact of magnetism on superconductivity. Recently, iron-based superconductors containing Eu (Eu-IBSs) have attracted attention as a new class of magnetic superconductors [1]. Eu-IBSs are characterized by higher critical temperature (Tc, up to 37 K) and Tm (~15-20 K) compared to other magnetic superconductors. The coexistence of superconductivity and magnetism extends over a wider range of temperatures and magnetic fields, allowing us to conduct experiments using various probes. Among Eu-IBSs, EuRbFe4As4 [2] shows highest Tc and upper critical field comparable to the non-magnetic CaKFe4As4 which has a strong vortex pinning ability [3], hence a unique interplay between pinned vortices and local magnetic moment is expected. Here we present the experimental observation of the ferromagnetic order induced by superconducting vortices in EuRbFe4As4. Although the ground state of the Eu2+ moments in EuRbFe4As4 is helimagnetism below Tm [4], neutron diffraction and magnetization experiments show a ferromagnetic hysteresis of the Eu2+ spin alignment. We demonstrate that the direction of the Eu2+ moments is dominated by the distribution of pinned vortices based on the critical state model. Moreover, we demonstrate the manipulation of spin texture by controlling the direction of superconducting vortices [5].

[1] S. Zapf and M. Dressel, Reports Prog. Phys. 80, 016501 (2017).
[2] K. Kawashima, et al., J. Phys. Soc. Jpn. 85, 064710 (2016).
[3] S. Ishida, et al. npj Quantum Materials 4, 27 (2019).
[4] K. Iida, et al., Phys. Rev. B 100, 014506 (2019).
[5] S. Ishida, et al. to be published in Proc. Natl. Acad. Sci. USA