We demonstrate a tunable bandgap from 2.32 eV to 2.09 eV in phase-pure BiFeO3 by controlling the particle size from 65 nm to 5 nm. Defect states due to oxygen and microstrain show a strong dependence on BiFeO3 particle size and have a significant effect on the shape of absorbance curves. Oxygen-defect induced microstrain and undercoordinated oxygen on the surface of BiFeO3 nanoparticles are demonstrated via HRTEM and XPS studies. Microstrain in the lattice leads to the reduction in rhombohedral distortion of BiFeO3 for particle sizes below 30 nm. The decrease in band gap with decreasing particle size is attributed to the competing effects of microstrain, oxygen defects, and Coulombic interactions.

We report on the electronic energy-level diagram of polycrystalline ${\text{BiFeO}}_3$ using the elemental, optical, and current-density-electric field $\left(J\text{−}E\right)$ characteristics. The elemental, electronic composition, and valence-band structure of ${\text{BiFeO}}_3$ ceramics were studied using x-ray photoelectron spectroscopy. The diffuse reflectance spectrum of a mixture of ${\text{BiFeO}}_3$ and ${\text{BaSO}}_4$, used as a standard, was recorded to test the Kubelka-Munk model. From the graph of the Kubelka-Munk function versus wavelength, two charge-transfer bands and two doubly degenerated $d\text{−}d$ transitions (${{}^6\text{A}}_{1\text{g}}\to {{}^4\text{T}}_{2\text{g}}$ and ${{}^6\text{A}}_{1\text{g}}\to {{}^4\text{T}}_{1\text{g}}$) were observed in polycrystalline ${\text{BiFeO}}_3$. The $J\text{−}E$ curves measured on the ${\text{BiFeO}}_3$ ceramics showed space-charge-limited conduction mechanism.

The ferrimagnetic spinel oxide ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ combines high Curie temperature and spin polarization with tunable electrical and magnetic properties, making it a promising functional material for spintronic devices. We have grown epitaxial ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films $\left(0\le x\le 0.9\right)$ on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an $\text{Ar}/{\text{O}}_2$ mixture by laser molecular beam epitaxy and systematically studied their structural, magnetotransport, and magnetic properties. We find that the electrical conductivity and the saturation magnetization can be tuned over a wide range (${10}^2\dots {10}^4{\Omega }^{-1}{\text{m}}^{-1}$ and $1.0\dots 3.2{\mu }_B/\text{f}\text{.u}\text{.}$ at room temperature) by Zn substitution and/or finite oxygen partial pressure during growth. Our extensive characterization of the films provides a clear picture of the underlying physics of the spinel ferrimagnet ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ with antiparallel Fe moments on the $A$ and $B$ sublattices: (i) Zn substitution removes both ${\text{Fe}}_A^{3+}$ moments from the $A$ sublattice and itinerant charge carriers from the $B$ sublattice; (ii) growth in finite oxygen partial pressure generates Fe vacancies on the $B$ sublattice also removing itinerant charge carriers; and (iii) application of both Zn substitution and excess oxygen results in a compensation effect as Zn substitution partially removes the Fe vacancies. Both electrical conduction and magnetism are determined by the density and hopping amplitude of the itinerant charge carriers on the $B$ sublattice, providing electrical conduction and ferromagnetic double exchange between the mixed-valent ${\text{Fe}}_B^{2+}/{\text{Fe}}_B^{3+}$ ions on the $B$ sublattice. A decrease (increase) in charge carrier density results in a weakening (strengthening) of double exchange and thereby a decrease (increase) in the conductivity and the saturation magnetization. This scenario is confirmed by the observation that the saturation magnetization scales with the longitudinal conductivity. The combination of tailored ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ films with semiconductor materials such as ZnO in multifunctional heterostructures seems to be particularly appealing.