The metastable iron oxide ϵ-Fe₂O₃ is rare but known for its magnetoelectric properties. While the more common alpha phase has been recognized for a long time as a suitable material for photoelectrochemical cells, its use is limited because of the electron-hole recombination problem when exposed to light. The indirect bandgap of the epsilon phase with its spontaneous polarization may offer a better potential for the application in photoelectrochemistry. Here, we report a detailed study of the electronic and structural features of the epsilon phase of iron oxide, its stability in thin films, and possible water dissociation reactions. Our studies are performed using density functional theory with a Hubbard-U correction. We observe that the stable ϵ-Fe₂O₃ surfaces favor the dissociation of water. The average difference in the energies of the states when water is adsorbed and when it is dissociated is roughly found to be −0.40 eV. Our results compare with the available experimental results where the epsilon phase is reported to be more efficient for the release of hydrogen from renewable oxygenates when exposed to sunlight.

The epsilon Fe₂O₃ phase of iron oxide has been studied to understand the spin structure and the magnetocrystalline anisotropy in the bulk and in thin films of ε-Fe₂O₃ and Co-doped ε-Fe₂O₃. The preferential magnetization direction in the nanoparticles of ε-Fe₂O₃ is along the a-axis [M. Gich et al., Chem. Mater. 18, 3889 (2006)]. Compared to the bulk band gap of 1.9 eV, the thin-film band gap is reduced to 1.3 eV in the Co-free films and to 0.7 eV in the film with partial Co substitution. The easy magnetization direction of the bulk and Co-free ε-Fe₂O₃ is along the c-axis, but it switches to the a-axis on Co substitution. All three systems exhibit in-plane anisotropies associated with the orthorhombic crystal structure of the oxide.

Bulk alloys and thin films of Fe-substituted Ti₃Co₅B₂ have been investigated by first-principle density-functional calculations. The series, which is of interest in the context of alnico magnetism and spin electronics, has been experimentally realized in nanostructures but not in the bulk. Our bulk calculations predict paramagnetism for Ti₃Co₅B₂, Ti₃Co₄FeB₂ and Ti₃CoFe₄B₂, whereas Ti₃Fe₅B₂ is predicted to be ferromagnetic. The thin films are all ferromagnetic, indicating that moment formation may be facilitated at nanostructural grain boundaries. One member of the thin-film series, namely Ti₃CoFe₄B₂, is half-metallic and exhibits perpendicular easy-axis magnetic anisotropy. The half-metallicity reflects the hybridization of the Ti, Fe and Co 3d orbitals, which causes a band gap in minority spin channel, and the limited equilibrium solubility of Fe in bulk Ti₃Co₅B₂ may be linked to the emerging half-metallicity due to Fe substitution.

First-principle calculations are used to determine the interface exchange coupling in cobalt-chromia thin films as a function of the electric field. Both bilayer Co/Cr₂O₃ and trilayer Co/Pt/Cr₂O₃ are investigated. The sign and magnitude of the interface exchange depend on the thickness of the cobalt layer, and oscillatory sign changes of the interface exchange are found in the trilayer system. The electric field dependence of the exchange may be exploited in voltage-controlled spin-electronics applications.

Disordered CoFeCrAl and CoFeCrSi0.5Al0.5 alloys have been investigated experimentally and by first-principle calculations. The melt-spun and annealed samples all exhibit Heusler-type superlattice peaks, but the peak intensities indicate a substantial degree of B2-type chemical disorder. Si substitution reduces the degree of this disorder. Our theoretical analysis also considers several types of antisite disorder (Fe-Co, Fe-Cr, Co-Cr) in Y-ordered CoFeCrAl and partial substitution of Si for Al. The substitution transforms the spin-gapless semiconductor CoFeCrAl into a half-metallic ferrimagnet and increases the half-metallic band gap by 0.12 eV. Compared CoFeCrAl, the moment of CoFeCrSi0.5Al0.5 is predicted to increase from 2.01 B to 2.50 B per formula unit, in good agreement with experiment.

The structural, magnetic, and electron-transport properties of Heusler-ordered CoFeCrAl thin films are investigated experimentally and theoretically. The films, sputtered onto MgO and having thicknesses of about 100 nm, exhibit virtually perfect single-crystalline epitaxy and a high degree of L21 chemical order. X-ray diffraction and transmission-electron microscopy show that the structure of the films is essentially of the L21 Heusler type. The films are ferrimagnetic, with a Curie temperature of about 390 K, and a net moment of 2 μB per formula unit. The room temperature resistivity is 175 μΩ cm; the carrier concentration and mobility determined from the low temperature (5 K) measurement are 1.2 × 1018 cm−3 and 33 cm2/V s, respectively. In contrast to the well-investigated Heusler alloys such as Co₂(Cr₁−xFe_x)Al, the CoFeCrAl system exhibits two main types of weak residual A2 disorder, namely, Co-Cr disorder and Fe-Cr disorder, the latter conserving half-metallicity. Point-contact Andreev reflection yields a lower bound for the spin polarization, 68% at 1.85 K, but our structural and magnetization analyses suggest that the spin polarization at the Fermi level is probably higher than 90%. The high resistivity, spin polarization, and Curie temperature are encouraging in the context of spin electronics.

The induced spin polarization of graphene on Cr₂O₃(001) is investigated using density-functional theory (DFT) and model calculations. The magnetic moment in graphene is a proximity effect and can be regarded as a second-order Stoner scenario, and similar mechanisms are likely realized for all graphene systems with an insulating magnetic substrate. In the absence of charge transfer, the magnetic moment would be quadratic in the exchange field, as contrasted to the usually encountered approximately linear dependence. The net magnetization of the graphene is small, of the order of 0.01 μB per atom, but the energy-dependent spin polarization exhibits pronounced peaks that have a disproportionally strong effect on the spin-polarized electron transport and are therefore important for spin-electronics applications.

The induced spin polarization of graphene on Cr₂O₃(001) is investigated using density-functional theory (DFT) and model calculations. The magnetic moment in graphene is a proximity effect and can be regarded as a second-order Stoner scenario, and similar mechanisms are likely realized for all graphene systems with an insulating magnetic substrate. In the absence of charge transfer, the magnetic moment would be quadratic in the exchange field, as contrasted to the usually encountered approximately linear dependence. The net magnetization of the graphene is small, of the order of 0.01 μB per atom, but the energy-dependent spin polarization exhibits pronounced peaks that have a disproportionally strong effect on the spin-polarized electron transport and are therefore important for spin-electronics applications.

Cobalt-chromia interface exchange interactions in the Co-on-Cr₂O₃(0001) system are investigated. Density-functional theory predicts the exchange coupling at the interface to be antiferromagnetic, in agreement with earlier experimental results. The spin-polarized photoemission spectra reveal both perpendicular and in-plane magnetization components, in the cobalt adlayer on chromia. A magnetization canted with respect to the surface normal, inferred from the presence of remnant spin polarization both in the plane of the cobalt film and along the surface normal may be understood as a micromagnetic canting effect involving magnetostatic self-interaction and exchange coupling between Co and Cr₂O₃.