Magnetite (Fe₃O₄) is of fundamental importance for Verwey transition near T_V = 125 K, below which a complex lattice distortion and electron orders occur. Verwey transition is suppressed by chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe₃O₄ nanoparticles leads to an intriguing variation of Verwey transition: an initial drop of T_V minima 70 K after 75 days and a followed recovery to 95 K after 160 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution from inhomogeneous to homogeneous doping level. This work demonstrates that slow oxidation of nanoparticles can give exquisite control and separation of homogeneous and inhomogeneous doping effects on Verwey transition and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.

Nature Communications 12, 6356 (2021)

Link1: https://www.nature.com/articles/s41467-021-26566-4