Size Dependence of Metal–Insulator Transition in Stoichiometric Fe₃O₄ Nanocrystals

Magnetite (Fe₃O₄) is one of the most actively studied materials with a famous metal–insulator transition (MIT), so-called the Verwey transition at around 123 K. Despite the recent progress in synthesis and characterization of Fe₃O₄ nanocrystals (NCs), it is still an open question how the Verwey transition changes on a nanometer scale. We herein report the systematic studies on size dependence of the Verwey transition of stoichiometric Fe₃O₄ NCs. We have successfully synthesized stoichiometric and uniform-sized Fe₃O₄ NCs with sizes ranging from 5 to 100 nm. These stoichiometric Fe₃O₄ NCs show the Verwey transition when they are characterized by conductance, magnetization, cryo-XRD, and heat capacity measurements. The Verwey transition is weakly size-dependent and becomes suppressed in NCs smaller than 20 nm before disappearing completely for less than 6 nm, which is a clear, yet highly interesting indication of a size effect of this well-known phenomena. Our current work will shed new light on this ages-old problem of Verwey transition.