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The magnetocrystalline anisotropy of pristine and Co-substituted ε-Fe2O3 is investigated by density functional calculations. The epsilon-iron oxide is the only polymorph of Fe2O3 magnetoelectric in its antiferromagnetic ground states other crystalline forms being α-Fe2O3 (hematite), β-Fe2O3, and γ-Fe2O3 (maghemite). The magnetizations of the four iron sublattices are antiferromagnetically aligned with slightly different magnetic moments resulting in a ferrimagnetic structure. Compared to the naturally occurring hematite and maghemite, bulk ε-Fe2O3 is difficult to prepare, but ε-Fe2O3 nanomaterials of different geometries and feature sizes have been fabricated. A coercivity of 20 kOe [2 T] was reported in nanocomposites of ε-Fe2O3, and an upper bound for the magnetic anisotropy constant K at a low temperature of ε-Fe2O3 is previously measured to be 0.1 MJ/m3. In the Co-substituted oxides, one octahedral or tetrahedral Fe atom per unit cell has been replaced by Co. The cobalt substitution substantially enhances magnetization and anisotropy.

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.