Iron-based magnetic superconductorsEuFeAs(Rb, Cs): natural superconductor-ferromagnet hybrids.

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 EuFeAswere found to render such a coexistence, primarily because of the Fe-3multi-orbitals which simultaneously satisfy the superconducting pairing and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductorsEuFeAs(= 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 betweenFeAsand EuFeAs, show SC in the FeAs bilayers at= 35-37 K and magnetic ordering at∼ 15 K in the sandwiched Eu-ion sheets. Below, the Euspins 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.