Near‐Field Optical Detection of Defect‐Stabilized Metallic Islands at the Verwey Transition in Fe3O4

Magnetite (Fe3O4) undergoes a metal‐to‐insulator transition (MIT) at the Verwey transition that is accompanied by significant structural distortions. This archetypical transition is extensively studied due to its highly correlated nature. Here, the formation of defect‐stabilized metallic islands is shown at the MIT that we directly image using high‐resolution scattering‐type scanning near‐field optical microscopy in the mid‐infrared (MIR) to terahertz (THz) spectral range. The transition to the insulating state on cooling is accompanied by the formation of remnant metallic islands just a few hundred nm in extent within less than 1 K of the MIT. Notably, these islands occur at similar places on repeated cooling cycles, suggesting the presence of pinning centers. Temperature‐dependent low‐frequency MIR and THz optical responses show a similarly sharp MIT, indicating a clear first‐order transition. On the other hand, as compared to the MIR signal, a reduction in the THz signal at temperatures above the MIT is indicative of short‐range ordering consistent with the formation of polarons. The findings highlight the importance of nanoscale optical imaging in understanding the electronic properties of strongly correlated materials.