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Mg–Cr ferrite has been studied with Mössbauer spectroscopy, x-ray diffraction, and vibrating sample magnetometer. The crystal structure for this system is spinel structure, and the lattice constant is in accord with Vegard’s law. The Mössbauer spectra consist of two six-line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Curie temperature decreases linearly with Cr concentration, suggesting the superexchange interaction Fe(A)–O–Fe(B) link is stronger than that for the Fe(A)–O–Cr(B) link. Debye temperatures for the A and B sites of MgFe1.9Cr0.1O4 are found to be θA=515±5 K and θB=265±5 K, respectively. Atomic migration of MgFe1.9Cr0.1O4 starts near 350 K and increases rapidly with increasing temperature to such a degree that 50% of the ferric ions at the A sites have moved over to the B sites by 600 K. The temperature dependence of both the magnetic hyperfine field and magnetization of MgFe1.9Cr0.1O4 is explained by the Néel theory of ferrimagnetism using three superexchange integrals: JA–B=−17.9kB, JA–A=1.9kB, and JB–B=7.1kB.

The effects of Fe doping on the magnetic and electronic properties of reduced TiO2−δ thin films have been investigated. Anatase Ti1−xFexO2−δ films exhibit a ferromagnetic behavior at room temperature for a certain range of Fe doping. Conversion electron Mössbauer spectroscopy measurements indicate that Fe2+ and Fe3+ ions coexist in the Fe-doped films, substituting the octahedral Ti4+ sites. The contribution of possible Fe3O4 clusters to the observed ferromagnetism is not likely to happen. The Ti1−xFexO2−δ films exhibit a p-type character by Hall effect measurements but the observed ferromagnetism turns out to be independent of the hole concentration. The observed ferromagnetism in the Ti1−xFexO2−δ films can be explained in terms of a direct ferromagnetic coupling between two neighboring Fe3+ ions via an electron trapped in oxygen vacancy nearby.

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Structural transformation and the related variation in magnetic and optical properties of Co 3−x Fe x O 4 thin films grown by a sol-gel method have been investigated as the Fe composition varies up to x=2. The normal spinel phase is dominant below x=0.55 and the inverse spinel phase grows as x increases further. Conversion electron Mössbauer spectroscopy (CEMS) measurements indicate that the normal spinel phase have octahedral Fe 3+ ions mostly while the inverse spinel phase contain octahedral Fe 2+ and tetrahedral Fe 3+ ions. For higher Fe composition (x >1.22), Co 2+ ions are found to substitute the octahedral Fe 2+ sites. The measured optical absorption spectra for the Co 3−x Fe x O 4 films by spectroscopic ellipsometry support the CEMS interpretation.