Disappearing of the Verwey transition in magnetite nanoparticles synthesized under a magnetic field: implications for the origin of charge ordering

It is observed that Fe3O4 nanoparticles exhibit a Verwey transition at approximately 120 K, in agreement with that observed in the corresponding bulk sample. While, the transition vanishes in the Fe3O4 nanoparticles formed under a 0.25 T magnetic field. It is found that the magnetic field induced oxidation of part of the Fe2+ ions on octahedral sites from lower spin states to higher spin states (Fe3+) during the formation of the nanoparticles, and then resulted in the vanishing of charge ordering between Fe2+ and Fe3+ on the octahedral sites below Verwey transition temperature (TV) due to lacking of Fe2+ in some layers.