Visualization of ferromagnetic domains in magnetic topological insulator/FeTe heterostructures in the superconducting state

The interfacial superconductivity of the topological insulator (TI)/antiferromagnetic FeTe heterostructures have attracted extensive attention in recent years. In this work, magnetic TI/FeTe heterostructures were synthesized and have a superconducting phase transition. In Hall resistance measurements, hysteresis loops can be observed below Curie temperature, but the resistance drops to zero when the sample enters the superconducting state. To confirm the ferromagnetism below superconducting temperature, we visualize ferromagnetic domains at 2.2 K using a cryogenic magnetic force microscope (MFM) with in-situ transport measurements. The evolution of domain nucleation and domain wall propagation under varying fields in zero resistance indicates a robust ferromagnetic order coexisting with superconductivity. Based on the domain population, we can construct ferromagnetic hysteresis loops in the superconducting state. The corresponding coercive fields are also in good agreement with reflective magnetic circular dichroism (RMCD) results.



**The MFM studies at Rutgers are supported by DOE BES under Award No. DE-SC0018153.