Wet H2 Reduction: A Robust Way of Converting α-Fe2O3 into Fe3O4 at the Nanoscale

Iron oxides are a ubiquitous class of magnetic materials, holding the promise of spintronic devices and magnetic hyperthermia based on their tunable magnetic and electric conducting properties. However, their high sensitivity towards oxidation/reduction environments makes them difficult to control in a reversible and scalable manner. Here we demonstrate two different approaches for reduction of antiferromagnetic insulator α-Fe2O3 thin films into ferrimagnetic conductor Fe3O4 thin films. The wet H2 reduction process is more efficient than the vacuum and inert gas atmosphere when producing nanocrystalline Fe3O4 thin films. We measure low-temperature physical properties and find that the wet H2 reduction process drives pronounced changes in magnetic anisotropy, Verwey transition, and surface spin canting-induced exchange bias in Fe3O4 thin films. We also explain how phase formation in nanocrystalline iron oxide thin films is different from their bulk equilibrium phase diagram.