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-3dmulti-orbitals which simultaneously satisfy the superconducting pairing 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 the interplay between SC and FM. The two sibling compounds, which can be viewed as an intergrowth or a hybrid betweenAFe₂As₂and EuFe₂As₂, show SC in the FeAs bilayers atT_c= 35-37 K and magnetic ordering atT_m∼ 15 K in the sandwiched Eu²⁺-ion sheets. BelowT_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.