RKKY-induced magnetic transition for pairs of magnetic atoms on proximitized superconducting surface

An atomic chain with helical spin texture proximitized by an s-wave superconductor can harbor topological superconductivity that gives rise to Majorana zero modes (MZMs) localized at its both ends. Understanding the exchange interaction between magnetic atoms on a superconducting surface is the key to realizing helical spin chains and MZMs. Here we use atomic manipulation to build magnetic Gd dimers on proximitized superconducting Bi(110) surface with a dilution refrigerator scanning tunneling microscope (STM). By measuring the in-gap Shiba states induced by magnetism of Gd dimers, we find the magnetic ground state of Gd dimers changes from out-of-plane ferromagnetic to in-plane antiferromagnetic and then to in-plane ferromagnetic phase as a function of the distance between two Gd atoms. The Shiba states splitting behavior in the in-plane ferromagnetic state suggests the exchange interaction between two Gd atoms is dominated by RKKY interaction. Finally, by taking magnetic anisotropy, spin-orbit coupling and RKKY interaction into account, we propose a theoretical model that can describe this system.