Iron-based magnetic superconductors AEuFe4As4 (A = Rb, Cs): natural superconductor–ferromagnet hybrids

Abstract

Abstract Superconductivity (SC) and ferromagnetism (FM) are normally antagonistic, and their coexistence in a single crystalline material appears to be very rare. Over a decade ago, the iron-based pnictides of doped EuFe2As2 were found to render such a coexistence, primarily because of the Fe–3d multi-orbitals which simultaneously satisfy the superconducting pairing and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductors AEuFe4As4 (A = Rb, Cs) provided an additional and complementary material basis for the study of the coexistence and the interplay between SC and FM. The two sibling compounds, which can be viewed as an intergrowth or a hybrid between AFe2As2 and EuFe2As2, show SC in the FeAs bilayers at T c = 35–37 K and magnetic ordering at T m ∼ 15 K in the sandwiched Eu2+-ion sheets. Below T m, the Eu2+ spins align ferromagnetically within each Eu plane, making the system as a natural atomic-thick superconductor–ferromagnet superlattice. This paper reviews the main research progress in the emerging topic during the past five years. An outlook for the future research opportunities is also presented.