Tunable Magnetic Phases at $\mathrm{Fe_{3}O_{4}/SrTiO_{3}}$ Oxide Interfaces

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 bulk-like 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