Coexistence of ferromagnetism and superconductivity in infinite-layer nickelates*

Numerous transport experiments have been performed on the recently discovered infinite-layer superconducting nickelates, yet scanning probe microscopy experiments have been lacking. We use a scanning SQUID (Superconducting QUantum Interference Device) to image surface magnetic response of several superconducting infinite-layer nickelates (RSrNiO2, R = La, Pr, Nd). Both ferromagnetic domains and diamagnetic screening are found to exist in the nickelate thin film. The ferromagnetic domains are attributed to Ni ions. The variation in magnetic signal suggests that an average domain contains thousands of Bohr magnetons, corresponding to an estimated size of a few thousand unit cells. The diamagnetic screening is inhomogeneous and locally suppressed in shapes of lines and crosses along crystal axis. In addition, we find traces of vortices buried under the ferromagnetic background. Our work emphasizes the difference between nickelates and cuprates, suggesting infinite layer nickelates could be a new class of superconductors.