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BiFeO3 (BFO) and Bi1−x La x Fe0.9Cr0.1O3 (x = 0.1–0.2, BLFC x=0.1–0.2) ceramics were prepared by modified solid-state reaction. The influence of La/Cr codoping on the microstructure, leakage, dielectric and magnetic properties of BFO was studied. It is found that the La/Cr codoping can induce a structural transformation from a rhombohedral (R3c) phase to a triclinic (P1) phase, suppress the formation of oxygen vacancies and modulate the spiral spin structure. With the increasing La concentration, the leakage current density can be diminished by over two orders of magnitude compared with undoped BFO, in a reasonable range for device applications. The dielectric loss follows almost the same trend as the leakage current, while the dielectric constant is raised to 400.8 at 100 kHz for x = 0.2, over eight times larger than that of undoped BFO. The substantially enhanced magnetization can be mainly attributed to the suppression of the spiral spin structure and a new ferromagnetic Fe3+–O2−–Cr3+ super-exchange interaction. More interestingly, abrupt changes are observed around x = 0.15, not only in microstructure but also in electrical and magnetic properties, which may indirectly reflect the coupling effect among them. The BLFC x=0.2 ceramic shows the highest magnetization and dielectric constant together with the lowest leakage current and dielectric loss. The results indicate that the La/Cr codoping can effectively improve the magnetic and electrical properties of BFO ceramic for potential applications in magnetoelectric devices.

Cu(Cr0.96−xMg0.04Mnx)O2−δ ceramics (0 ≤ x ≤ 15 at.%) were prepared by solid-state reaction. The influence of Mg acceptor codoping on microstructure, magnetic, and optical properties was studied. The codoping with Mg acceptors and the resulting lattice expansion are both favorable for the enhancement of hole density. Furthermore, the lower valence of Mg2+ compared with Cr3+ enables the mixed valence state of Mn ions: Mn3+ and Mn4+, and hence induces the hole-mediated double-exchange mechanism not only between Mn3+ and Cr3+ but also between Mn3+ and Mn4+. The coexistence of two ferromagnetic interactions and the high hole density are responsible for the improved saturation magnetization and Curie temperature. For photoluminescence, an unusual near-infrared emission at 1.5 eV is observed in the Mn-contained samples and can be attributed to the internal transition between the e and t2 states of the Mn3+ impurity band. The emission intensity is primarily affected by two factors: the Mn3+ concentration and the position of the Fermi level. The results show that codoping with Mg acceptors is an effective way to strengthen the hole-mediated ferromagnetic interactions in CuCrO2 delafossite.

Cu(Cr1−xMnx)O2 delafossite thin films (0 ⩽ x ⩽ 15 at.%), with a c-axis quasi-epitaxial orientation, were prepared by pulsed laser deposition. The effects of Mn content on microstructure and physical properties were investigated. Both the variation of lattice constant c and X-ray photoelectron spectroscopy reveal that the Mn ions substitute for Cr3+ as Mn3+ and Mn4+. The proportion of Mn4+ is gradually increased with the Mn doping, leading to the changes of hole density and mobility. As a result, the p-type conductivity is decreased at first and then increased. The conduction mechanism is thermal activation between 130 and 300 K. It is indicated that the PLD method and the Mn3+–O–Mn4+ and Mn3+–O–Cr3+ double-exchange interactions are favorable to the high hole density and mobility, respectively. Meanwhile, the double-exchange interactions produce the near-room-temperature ferromagnetism. The saturation magnetization and Curie temperature are gradually increased with the Mn content. For optical properties, all thin films have a comparatively high transmittance for visible light, with the highest value 70% for x = 15 at.% at the wavelength of 750 nm. As increasing the Mn content, the transparency and direct optical bandgap exhibit the similar trend as the hole density. With the relatively balanced electrical, magnetic and optical properties, these pulsed-laser-deposition-grown Cu(Cr1−xMnx)O2 thin films are expected to become a promising p-type transparent diluted magnetic semiconductor material.

Fe-doped CuCrO 2 delafossite thin films, with a c -axis quasi-epitaxial orientation, were prepared by pulsed laser deposition, exhibiting reasonably good p-type conductivity, ferromagnetism with T C well above room temperature, and comparatively high transparency in the visible range. The influence of Fe concentration (0–15 at%) on microstructure and physical properties was investigated. The substitution of Fe 3+ for Cr 3+ induces the lattice expansion, which is favorable for the oxygen insertion and thus produces the higher hole concentration. Accordingly the p-type conductivity is remarkably improved to 28 S cm −1 . Moreover, the introduction of Fe 3+ leads to the hole-mediated Fe 3+ –Cr 3+ super-exchange coupling responsible for the room-temperature ferromagnetism. The saturation magnetization is enhanced as the Fe concentration increases, with Curie temperature ⩾550 K. Nevertheless, the transparency in the visible range does not profit from the Fe doping. The transmittance is diminished from 75% for x = 0 to 65% for x = 15 at% at the wavelength of 750 nm, which can be interpreted as the strengthened scattering of photons by charge carriers. The direct optical bandgap is modulated by 3.8% via the Fe addition. The results show that the substitution of iron for chromium is an effective way to achieve high-performance CuCrO 2 thin films which are supposed to become a promising p-type transparent diluted magnetic semiconductor material.

The preferred c-oriented Cu(Cr1−xFex)O2 ceramics (0 ⩽ x ⩽ 15 at.%) were prepared by solid-state reaction. The effects of Fe concentration on microstructure, morphology and magnetism were investigated. All the samples have a pure 3R-CuCrO2 delafossite structure, with a stress-induced preferential orientation. The lattice expansion supports the Fe entrance into the Cr sublattice in the form of Fe3+, which is further certified by the X-ray photoelectron spectra and the Mossbauer spectra. The Mossbauer study also reveals a preferential growth orientation and an increase of Fe–O bond lengths with the Fe addition, matching well with the X-ray diffraction analysis. Ferromagnetism is achieved in the Fe-doped samples with Curie temperatures higher than 246 K, which is expected to originate from the hole-mediated Fe3+–Cr3+ super-exchange interaction. The saturation magnetization of this CuMO2 delafossite (M = Cr, Fe) is decreased almost linearly with the increasing Fe concentration, due to the combined influence of the number of the M–M pairs, the M–M distances and the hole concentration. The present work indicates that Fe-doped CuCrO2 is a dilute magnetic semiconductor and Curie temperature can be remarkably enhanced to close to room temperature by Fe doping.

Using p-type transparent conducting CuCrO 2 as a host material, (Cu 1− x Fe x )CrO 2 (0 ≤ x ≤ 6 at.%) and Cu(Cr 1− y Fe y )O 2 (0 ≤ y ≤ 6 at.%) nanocrystals were prepared by combining solid-state reaction and ball milling. The microstructure, morphology, optical and magnetic properties were characterized by X-ray diffraction, atomic force microscopy, diffuse reflectance spectroscopy and vibrating sample magnetometer. It is found that all the samples have a single CuCrO 2 delafossite structure. The variations in lattice constants support the Fe entrance into the Cu and Cr sublattices, respectively, in (Cu 1− x Fe x )CrO 2 and Cu(Cr 1− y Fe y )O 2 . The gradual increase of optical bandgap with the Fe content and the low-temperature ferromagnetism are observed in two series of samples. The present work indicates that Fe-doped CuCrO 2 is a dilute magnetic semiconductor and the Curie temperature can be expected to enhance by the optical bandgap modulation.

The A-site substituted BaTiO 3 ceramics were prepared by solid-state reaction via partial substitution of Fe for Ba 2+ . By comparison with the B-site substituted sample made under similar conditions, the effect of Fe doping site on microstructure and magnetism was investigated using X-ray diffraction, Mossbauer spectroscopy and vibrating sample magnetometer. It is found that A-site substitution can be realized to a certain extent at 7 at% Fe addition, whereas impurities are observed at higher Fe concentrations. In the nominal (Ba 0.93 Fe 0.07 )TiO 3 sample, the Fe ions are present as Fe 2+ and Fe 3+ , respectively, replacing A-site Ba 2+ and octahedral B-site Ti 4+ in hexagonal perovskite lattice. The double-exchange Fe 2+ –O 2− –Fe 3+ interactions produce ferromagnetism well above room temperature, but the saturation magnetization and the Curie temperature are both obviously lower than those for B-site substitution due to different magnetic exchange mechanisms. In the B-site substituted sample Ba(Ti 0.93 Fe 0.07 )O 3 , the super-exchange interactions between Fe 3+ on pentahedral and octahedral Ti 4+ sites are responsible for ferromagnetism. These results mean that B-site substitution is a better way for Fe-doped BaTiO 3 system to obtain high-Curie-temperature ferromagnetism. Moreover, increasing pre-sintering time can further improve the magnetism of B-site substituted samples, through which the saturation magnetization for Ba(Ti 0.93 Fe 0.07 )O 3 is enhanced ∼6 times.

Fe-doped (Ba 1− x Sr x )TiO 3 ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤ x ≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe 3+ on pentahedral and octahedral Ti sites mediated by the O 2− ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe 3+ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba 0.25 Sr 0.75 )TiO 3 , is an order of magnitude higher than that of the Fe-doped BaTiO 3 system with similar Fe concentration and preparation conditions, which may indicate (Ba 1− x Sr x )TiO 3 as a more suitable matrix material for multiferroic research.

The (Ba 0.8 K 0.2 )(Ti 0.3 Fe 0.7 )O 3 ceramic was prepared by solid-state reaction, and post-annealed in oxygen ambient. By comparison with Ba(Ti 0.3 Fe 0.7 )O 3 made under identical conditions, the effect of non-isovalent A-site substitution of K + on microstructure and magnetism of as-prepared and annealed Ba(Ti 0.3 Fe 0.7 )O 3 samples was investigated using X-ray diffraction, Mossbauer spectroscopy, vibrating sample magnetometer and iodometric titration. It is found that all samples have a single 6H-BaTiO 3 -type hexagonal perovskite structure without any impurities detected, regardless of A-site K + substitution or annealing. In the as-prepared state, non-isovalent A-site substitution of K + induces the variation in Fe occupational site, resulting in the disappearance of room-temperature ferromagnetism. The super-exchange interactions of Fe 3+ at tetrahedral and octahedral Ti sites determine the paramagnetism of (Ba 0.8 K 0.2 )(Ti 0.3 Fe 0.7 )O 3 . During the O 2 annealing process, the presence of Fe 4+ , an unusual valence for iron, besides Fe 3+ is observed, both distributed over octahedral Ti site. By A-site substitution of K + with a lower valence than Ba 2+ , the charge compensation mechanism is further enhanced, and thus more Fe 3+ ions are oxidized to Fe 4+ in annealed (Ba 0.8 K 0.2 )(Ti 0.3 Fe 0.7 )O 3 . Consequently, the ferromagnetic Fe 4+ –O 2− –Fe 4+ super-exchange interactions are strengthened, which leads not only to a paramagnetism–ferromagnetism transition but also to a higher saturation magnetization compared with annealed Ba(Ti 0.3 Fe 0.7 )O 3 .

Fe-doped (Ba0.8Sr0.2)TiO3 ceramic was prepared by solid-state reaction, and room-temperature ferromagnetism was achieved via vacuum annealing. The effect of annealing on microstructure, magnetic properties and exchange mechanism was investigated using X-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometer. The as-prepared and annealed samples are shown to be monophasic, crystallizing in a 6H-BaTiO3-type hexagonal perovskite structure. The paramagnetism is exhibited in the as-prepared state, arising from the super-exchange interactions of Fe3+ on tetrahedral and octahedral Ti sites. On the other hand, vacuum annealing is proved to be a feasible way for Fe-doped (Ba1−xSrx)TiO3 to obtain ferromagnetism via modifying the Fe occupational distribution. The super-exchange interactions between Fe3+ on pentahedral and octahedral Ti sites lead to the room-temperature ferromagnetism observed in the annealed sample.

Ba(Ti 0.65 M 0.05 Fe 0.3 )O 3 ( M = Cr, Mn and Ni) ceramics were prepared by solid-state reaction, and post-annealed in vacuum and oxygen, respectively. Mossbauer spectroscopy was used to investigate the local environments of Fe atoms. Combined with the configuration of the other transition metal M , the mechanisms responsible for magnetism were discussed for different samples. The results show that all samples are single-phase with a 6H–BaTiO 3 -type hexagonal perovskite structure. The as-prepared ceramics are all paramagnetic, while the annealed ones are all ferromagnetic at room temperature. The origin of ferromagnetism changes with doping element and annealing atmosphere, including the double-exchange mechanism of transition metal M with mixed valences, the super-exchange mechanism of Fe 3+ in different occupational sites, and the super-exchange mechanism of Fe 4+ –O 2− –Fe 4+ .

Ba(Ti 0.3 Fe 0.7 )O 3 ceramic was prepared by solid-state reaction and post-annealed in vacuum and oxygen, respectively. The as-prepared and annealed samples are all single-phase, crystallizing in a 6H–BaTiO 3 -type hexagonal perovskite structure. Room-temperature ferromagnetism is exhibited in all ceramics. For the as-prepared sample, the super-exchange interactions of Fe 3+ in different occupational sites (pentahedral and octahedral sites) are expected to produce the ferromagnetism observed. After annealing in vacuum, the magnetization is reduced while the exchange mechanism remains unchanged. On the contrary, O 2 annealing can effectively enhance the magnetization due to the presence of Fe 4+ , an unusual valence for iron. The simultaneous presence of Fe 3+ and Fe 4+ allows new exchange mechanism responsible for the ferromagnetic interaction. The exchange couplings of Fe ions with mixed valences (Fe 3+ and Fe 4+ ) determine the magnetic behavior.

Microstructure and magnetic properties of crystalline Ce 1 Y 2 Fe 5 O 12 thin films prepared on GGG and on SiO 2 /Si substrates by pulsed laser deposition were studied. The results show that highly textured Ce 1 Y 2 Fe 5 O 12 film with (4 4 4) preferred orientation prepared on GGG (1 1 1) shows strong paramagnetism superimposed by a weak ferromagnetism. However, polycrystalline Ce 1 Y 2 Fe 5 O 12 thin films on SiO 2 /Si, which can only be obtained after post-annealing, show strong ferromagnetism with easy axis of magnetization lying in the plane of the film. With post-annealing temperature increasing, CeO 2 segregates from Ce 1 Y 2 Fe 5 O 12 ; then YIG continues to be decomposed, forming Fe 2 O 3 . Consequently, the saturation magnetization of Ce 1 Y 2 Fe 5 O 12 films decreases first and then increases correspondingly, which indicates that the magnetic properties of Ce 1 Y 2 Fe 5 O 12 films are mainly related to the microstructure.

Ba(Ti 1− x Fe x )O 3 ceramics ( x =7, 30 and 70 at%) were prepared by solid-state reaction. All samples are single-phase with 6H-BaTiO 3 -type hexagonal perovskite structure. Mossbauer spectra show all Fe atoms to be present as Fe 3+ in BaTiO 3 lattice, occupying M 1 octahedral and pentahedral sites. Room-temperature ferromagnetism is exhibited and saturation magnetization gradually decreases with increasing Fe content. The observed ferromagnetism is considered to be an intrinsic property of Ba(Ti 1− x Fe x )O 3 , originating from super-exchange interactions between Fe 3+ in different occupational sites associated with oxygen vacancies. The variation in magnetization with Fe content is related to the ratio of pentahedral to octahedral sites and oxygen vacancies.

X-ray diffraction (XRD), 57Fe Mossbauer and magnetic measurements were performed on mechanically alloyed FexCr90−xMn10 (x=18 and 35) alloys. XRD and 57Fe Mossbauer spectra show the systems are non-magnetic BCC phase. The irreversibility between field-cooled and zero-field-cooled magnetization curves at relative high temperature is due to the blocking of some single particles. A correlation between the Fe concentration and Tmax of ZFC curves can be explained by the fine Fe-rich clusters which are embedded in a paramagnetic solid solution. The Fe35Cr55Mn10 alloy exhibits two type of blocking/freezing in an ensemble of nanoparticles. With the decreasing of temperature, the Fe18Cr72Mn10 alloy shows spin-glass-like behavior and the Fe35Cr55Mn10 alloy goes through two magnetic transitions from paramagnetic phase to ferromagnetic phase and then to re-entrant spin-glass state.

Ge 99.04 Mn 0.96 thin film was fabricated by thermal evaporation of Mn doped GeO 2 ceramic film under hydrogen atmosphere. Secondary phases were not detected by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) analyses. The film is p type. Room-temperature ferromagnetism was detected in the film. The ferromagnetic behavior may arise from alignment of the bound magnetic polarons (BMP) mediated by the localized holes in the system as well as ultra small secondary phases unable to be detected by XRD and HRTEM analyses.