We demonstrate the emergence and control of magnetic phases between magnetite (Fe 3 O 4 ), a ferrimagnetic halfmetal, and SrTiO 3 , a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition (T V ) was detected to persist from bulklike down to ultrathin Fe 3 O 4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe 2+ ions with decreasing film thickness, accompanied by an increase of Fe 3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe 2 O 3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γ-Fe 2 O 3 that finally leads to stoichiometric and ferrimagnetic Fe 3 O 4 /SrTiO 3 (001) heterointerfaces.
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