Iron-based magnetic superconductorsAEuFe4As4(A= 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 EuFe2As2were found to render such a coexistence primarily because of the Fe-3dmultiorbitals which simultaneously satisfy the superconducting pairing of Fe-3delectrons and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductorsAEuFe4As4(A= Rb, Cs) provided an additional and complementary material basis for the study of the coexistence and interplay between SC and FM. The two sibling compounds, which can be viewed as the intergrowth or hybrid betweenAFe2As2and EuFe2As2, show SC in the FeAs bilayers atTc= 35 - 37 K, followed by a magnetic ordering atTm∼ 15 K in the sandwiched Eu2+-ion sheets. BelowTm, 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, and an outlook for the future research opportunities is also presented.