Direct evidence of Fe²⁺-Fe³⁺ charge ordering in the ferrimagnetic hematite-ilmenite Fe_1.35Ti_0.65O _3-δ thin films

In this Letter we highlight direct experimental evidence of Fe2 ⁺-Fe3⁺ charge ordering at room temperature in hematite-ilmenite Fe_1.35Ti_0.65O_3-δ epitaxial thin films grown by pulsed laser deposition, using aberration-corrected scanning transmission electron microscopy coupled to high-resolution energy electron-loss spectroscopy. These advanced spectromicroscopy techniques demonstrate a strong modulation of the Fe2 ⁺ valence state along the c axis. Density functional theory calculations provide crucial information on the key role of oxygen vacancies in the observed charge distributions. Their presence at significant levels leads to the localization of extra electrons onto reduced Fe2⁺ sites, while Ti remains solely +4. The magnetic and transport properties of these films are reviewed in the light of the present results regarding their ferrimagnetic character correlated with the Fe2⁺ modulation and their semiconducting behavior interpreted by an Efros-Shklovskii variable-range hopping conduction regime via Fe2⁺ and Fe3⁺ centers. The experimental evidence of only one type of mixed valence state, i.e., Fe2 ⁺ and Fe3⁺, in the Fe_2-xTi_xO _3-δ system will thus help to interpret further the origin of its geomagnetic properties and to illuminate fundamental issues regarding its spintronic potential.