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Manganese ferrite (Mn x Fe 3− x O 4 ) thin films have been prepared by a sol–gel method. The samples ( x ≤1.25) are polycrystalline containing well-defined grains and maintain cubic spinel structure with increasing lattice constant with x . The substituting Mn ions were found to have multi-valence, +2 and +3, by X-ray photoelectron spectroscopy. The saturation magnetization of the Mn-substituted films measured by vibrating sample magnetometry was found to increase from that of Fe 3 O 4 at low x and then gradually decreases as x increases further. Such magnetic behavior can be explained in terms of the dominance of Mn 2+ species over Mn 3+ at low x and the gradual decrease in the population ratio Mn 2+ /Mn 3+ as x increases. The observed decrease in the coercivity with increasing x implies the increase in octahedral Mn 2+ population. The Mn-substituted samples exhibit magnetoresistance (MR) effect the maximum intensity of which is gradually reduced with x from that of Fe 3 O 4 . All samples show increasing MR with increasing external field ( H ), while their magnetization curves start to saturate near H =2 kOe. Such increasing MR with H can be explained in terms of the tunneling of spin-polarized carriers across grain boundaries. The reduction in the MR intensity with x can be partly explained in terms of the decrease in spin-polarized carrier density associated with octahedral occupation of Mn 2+ ions.

Room-temperature ferromagnetic properties of Mn-doped reduced titanium dioxide ( Ti 1 - x Mn x O 2 - δ ) thin films with rutile structure synthesized by a sol–gel method have been detected for a limited range of Mn composition ( x ) . The Ti 1 - x Mn x O 2 - δ films were found to be p-type semiconducting with hole concentration near 10 19 cm - 3 . The observed ferromagnetism in the Ti 1 - x Mn x O 2 - δ films is not related to the hole concentration but related to x. The room-temperature ferromagnetism is attributable to magnetic polaron formed by trapped electron in oxygen vacancy and magnetic ions around it. Thus, the existence of oxygen vacancies is necessary for the room-temperature ferromagnetism.

Magnetic and optical properties of FexCo3−xO4 thin films grown by sol–gel method have been investigated as the Fe composition (x) increases from 0 to 2. X-ray diffraction measurements revealed that the normal- and inverse-spinel phases coexist for 0.76 ⩽ x ⩽ 0.93 . The normal-spinel phase is dominant below x = 0.76 while the inverse-spinel phase above x = 0.93 . The lattice constant of the inverse-spinel phase is found to be larger than that of the normal-spinel phase. For both phases the lattice constant increases with increasing x. The FexCo3−xO4 films containing the inverse-spinel phase exhibit net magnetization that increases with increasing x. Conversion electron Mossbauer spectrum measured on the x = 0.93 sample showed that Fe2+ ions prefer the octahedral sites, indicating the formation of the inverse-spinel phase. Analysis on the measured optical absorption spectra for the samples by spectroscopic ellipsometry indicates a dominance of the normal-spinel phase for low x in which Fe3+ ions mostly occupy the octahedral sites. Observation of a crystal-field transition at 1.6 eV originating from tetrahedral Fe3+ ion confirms the existence of the inverse-spinel phase for high x.

Magneto-optical properties of spinel ferrite CuFe2O4 have been investigated in comparison with those of isostructural Fe3O4. The Kerr angle and ellipticity of the ferrites were measured by magneto-optic Kerr effect spectroscopy in the 1.5– 5 eV region. It is found that a strong 2.1-eV absorption peak for Fe3O4, due to a d–d intervalence charge-transfer (IVCT) transition in the octahedral sites, Fe2+(t2g)→Fe3+(eg), disappears for CuFe2O4. It gives an evidence that the octahedral Fe2+ sites are well substituted by Cu2+ ions in CuFe2O4. Absorption structures at about 2.5, 3.5, and 3.9 eV observed for both CuFe2O4 and Fe3O4 are assigned to inter-sublattice charge-transfer transitions between the tetrahedral and the octahedral Fe3+ ions. Two peaks observed at about 4.3 and 4.7 eV for CuFe2O4 are interpreted as due to Jahn–Teller-coupled IVCT transitions, Cu2+(eg)→Fe3+(t2g), in the octahedral sites. The structure at about 1.8 eV observed for CuFe2O4 is attributable to Jahn–Teller-coupled d–d crystal-field transitions, 2 E g → 2 T 2 g , in the octahedral Cu2+ ion.

Optical properties of DO 3 -ordered Fe 3 Co and Co 3 Fe compounds were investigated by spectroscopic ellipsometry in the 1.5–5.2 eV photon energy region. A broad interband absorption structure exists around 2 eV in the optical conductivity spectra of the compounds. The absorption structure of Co 3 Fe is located at lower energies than that of Fe 3 Co by about 0.5 eV. The calculated spin-polarized band structures of the compounds by the linear-muffin-tin orbitals method revealed that the Fe- and Co-like d states disperse in the same energy region because of hybridization. The theoretically calculated optical absorption structure of Fe 3 Co shows a good agreement with the experimental result while that of Co 3 Fe is located at higher energies than the experimental result by about 0.3 eV. Such discrepancy is interpreted as due to the self-energy shift of the interband transitions involving correlated Co-like d electrons.

Magnetic and electronic properties of Fe-doped anatase TiO 2− δ thin films grown by a sol–gel method have been investigated by vibrating-sample magnetometry (VSM), conversion electron Mossbauer spectroscopy (CEMS), and Hall effect measurements. VSM measurements revealed that the anatase Ti 1− x Fe x O 2− δ films exhibit ferromagnetic behavior at room temperature for a certain range of x . CEMS spectra revealed that Fe 2+ and Fe 3+ ions coexist, substituting the octahedral Ti 4+ sites. By appropriate Fe doping, the Ti 1− x Fe x O 2− δ films exhibited p-type character but the observed room-temperature ferromagnetism turned out to be independent of the hole concentration.

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe 2 O 4 have been investigated. Mn x Co 1− x Fe 2 O 4 and Mn y CoFe 2− y O 4 thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn x Co 1− x Fe 2 O 4 indicates that Mn 2+ ions substitute the octahedral Co 2+ sites. Conversion electron Mossbauer spectroscopy data indicate that a fraction of octahedral Co 2+ ions exchange sites with tetrahedral Fe 3+ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn x Co 1− x Fe 2 O 4 and Mn y CoFe 2− y O 4 films compared to that of the CoFe 2 O 4 film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co 2+ migration.

Distribution of transition-metal ions and the related magnetic and electronic properties of vanadium-substituted magnetite (V x Fe 3− x O 4 ) thin films have been investigated. The V x Fe 3− x O 4 films were found to maintain cubic structure up to x =1.0 with little change in the lattice constant. Analysis on 2p core-level XPS spectra of V indicates that V ions exist with a valence of +3 mostly and small +2 population in the compound. Analysis on the Mossbauer spectra indicates that V ions mainly substitute the octahedral sites. VSM data reveal that the magnetization of the V x Fe 3− x O 4 films increases at low x compared to that of Fe 3 O 4 and then gradually decreases as x increases. The coercivity of the samples is found to increase with x , attributable to the increase of anisotropy by the substitution of V 2+ (d 3 ) ions at the octahedral sites.

Effects of Mn and Cr substitution for Co on crystallographic and magnetic properties of inverse-spinel CoFe 2 O 4 thin films were investigated. The crystal structure of the samples remain cubic for x a 0 ) increasing with x for Mn doping and remaining constant for Cr doping. Tetrahedral Fe 2+ ions were detected in Cr x Co 1− x Fe 2 O 4 by Mossbauer spectroscopy while no such ions existed in Mn x Co 1− x Fe 2 O 4 . The appearance of the tetrahedral Fe 2+ ions can be explained in terms of the Cr 3+ substitution for the octahedral Co 2+ sites with the resultant charge imbalance being compensated by a reduction of the tetrahedral Fe 3+ into Fe 2+ . The observed variation in a 0 and magnetic properties can be partly explained in terms of Mn 2+ and Cr 3+ substitution of octahedral sites in Mn x Co 1− x Fe 2 O 4 and Cr x Co 1− x Fe 2 O 4 , respectively.