Interface correlated exchange bias effect in epitaxial Fe₃O₄ thin films grown on SrTiO₃ substrates

We report exchange bias effect in Fe₃O₄ films epitaxially grown on SrTiO₃ substrates. This effect is related to the formation of Ti³⁺-vacancy complexes at the surface of SrTiO₃ in ultrahigh vacuum that in turn triggers the growth of a thin antiferromagnetic (AFM) FeO layer (∼5 nm) at the interface. The picture of antiferromagnetic FeO interacting with native ferrimagnetic Fe₃O₄ matrix reasonably accounts for this anomalous magnetic behavior. With increasing film thickness from 17 to 43 nm, the exchange bias effect and the magnetization anomaly associated with the AFM phase transition of the FeO layer are progressively weakened due to the increase in the volume fraction of the Fe₃O₄ phase, indicating the interfacial nature of the exchange coupling. Our results highlight the important role of interface engineering in controlling the magnetic properties of iron oxide thin films.