Direct Evidence of ${\mathrm{Fe}}^{2\mathbf{+}}$-${\mathrm{Fe}}^{3\mathbf{+}}$ Charge Ordering in the Ferrimagnetic Hematite-Ilmenite ${\mathrm{Fe}}_{1.35}{\mathrm{Ti}}_{0.65}{\mathbf{O}}_{3\mathbf{-}\delta }$ Thin Films

In this Letter we highlight direct experimental evidence of ${\mathrm{Fe}}^{2+}$-${\mathrm{Fe}}^{3+}$ charge ordering at room temperature in hematite-ilmenite ${\mathrm{Fe}}_{1.35}{\mathrm{Ti}}_{0.65}{\mathrm{O}}_{3-\delta }$ 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 ${\mathrm{Fe}}^{2+}$ 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 ${\mathrm{Fe}}^{2+}$ 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 ${\mathrm{Fe}}^{2+}$ modulation and their semiconducting behavior interpreted by an Efros-Shklovskii variable-range hopping conduction regime via ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ centers. The experimental evidence of only one type of mixed valence state, i.e., ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$, in the ${\mathrm{Fe}}_{2-x}{\mathrm{Ti}}_x{\mathrm{O}}_{3-\delta }$ system will thus help to interpret further the origin of its geomagnetic properties and to illuminate fundamental issues regarding its spintronic potential.

Figure 1

(a) HAADF image of the region of interest. (b) Reconstructed map combining the $\mathrm{O}K$, $\mathrm{Ti}{L}_{2,3}$, and $\mathrm{Fe}{L}_{2,3}$ edges (red, green, and blue), (c), (d), and (e) reconstructed maps of the ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ spectral weights and the ${\mathrm{Fe}}^{2+}:{\mathrm{Fe}}^{3+}$ ratio, respectively. (f) Examples of linear combinations (red line) of ${\mathrm{Fe}}^{2+}$ (siderite—blue line) and ${\mathrm{Fe}}^{3+}$ (hematite-green line) energy-loss near-edge structure reference spectra fitting experimental ones (black line) acquired on an Fe-rich (site $A$) or a mixed Fe-Ti ($B$) site (each single spectrum was acquired in 100 ms). White arrows in Fig 1a indicate the scan direction. Fe, Ti, and O atoms of the structural model (inner upper schemes) are illustrated in violet, gray, and red, respectively.

Figure 2

(a) Fe oxidation state distributions in ${\mathrm{Fe}}_{1.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ (Ti atoms are all ${\mathrm{Ti}}^{4+}$), (b) ${\mathrm{Fe}}_{1.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3-\delta }$ model including a single oxygen vacancy, (c) distribution of Fe oxidation states in the two layers around the oxygen vacancy site. In all figures, Ti atoms are labeled in gray and O in red. In (b) Fe atoms are in violet while in (a) and (c) ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ are in blue and green, respectively. Additionally in (a) and (c), ${\mathrm{Fe}}^{3+}$ are marked by a quarter circle to distinguished from ${\mathrm{Fe}}^{2+}$ cations.

Figure 3

(a) Magnetic field dependence of magnetization at 10 and 300 K (circles and squares, respectively) for the ${\mathrm{Fe}}_{1.35}{\mathrm{Ti}}_{0.65}{\mathrm{O}}_{3-\delta }$ films, inset: temperature dependence of the saturation magnetization (magnetic field along the film plane). (b) Temperature dependence of the dc electrical conductivity (black dots) fitted with an Efros-Shklovskii (ES) model at $T>100\mathrm{K}$ (red line), the upper $x$ axis is the temperature (in K).