Intervening Oxygen Enabled Magnetic Moment Modulation in Spinel Nanostructures

Oxygen vacancies and lattice oxygen on the surface of nanoparticles play crucial roles in the magnetism of transition metal oxides. A fundamental understanding of the interactions between the 2p orbitals of oxygen anions and the 3d orbitals of metal cations is of immense interest for applications due to specific electronic, magnetic, and chemical properties. The spinel nanostructures Co3O4, Fe3O4, and CoFe2O4 present a unique model to alter coordination and bonding due to d electrons and oxygen 2p states. We performed complementary experimental techniques and corresponding model calculations to show that the overall net magnetism differs significantly between the O2-rich and -deficient environments. It was found that in monometallic spinel oxides (e.g., Co3O4 and Fe3O4), the changes in the spin moment are relatively small (∼0.08 μB per formula unit) compared to those in the bimetallic spinel oxide CoFe2O4 (∼0.40 μB per formula unit). Our study illustrates that the O2-rich/deficient conditions alter the exchange of Oh sites and adjust the metal ion moments so as to impact the overall magnetism.