Direct Mapping of Phase Separation across the Metal-Insulator Transition of NdNiO₃

Perovskite rare-earth nickelates RNiO₃ are prototype correlated oxides displaying a metal-insulator transition (MIT) at a temperature tunable by the ionic radius of the rare-earth R. Although its precise origin remains a debated topic, the MIT can be exploited in various types of applications, notably for resistive switching and neuromorphic computation. So far, the MIT has been mostly studied by macroscopic techniques, and insights into its nanoscale mechanisms were only provided recently by X-ray photoemission electron microscopy through absorption line shifts, used as an indirect proxy to the resistive state. Here, we directly image the local resistance of NdNiO₃ thin films across their first-order MIT using conductive-atomic force microscopy. Our resistance maps reveal the nucleation of ∼100-300 nm metallic domains in the insulating state that grow and percolate as temperature increases. We discuss the resistance contrast mechanism, analyze the microscopy and transport data within a percolation model, and propose experiments to harness this mesoscopic electronic texture in devices.