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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(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.

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.

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.

Hf x Zn 1− x O thin films ( x =3, 7, 10 and 15 mol%) were deposited on Si (1 0 0) using pulsed laser deposition. The influence of the Hf concentration on the microstructure and optical properties of the films was studied. It is found that Hf ions can be effectively doped into ZnO and all films crystallize in the hexagonal wurtzite structure with a preferred c -axis orientation. The lattice constants of Hf x Zn 1− x O films increase with the Hf contents. Two ultraviolet peaks centered at about 364 and 380 nm coexist in the fluorescent spectra. With increasing the Hf contents, the intensity of fluorescent peaks enhances remarkably. At the same time the energy gaps gradually increase, while the positions of ultraviolet peaks remain unchanged. The mechanism of luminescent emission for Hf x Zn 1− x O films was discussed.

Amorphous Ce1Y2Fe5O12 (Ce:YIG) thin films deposited on single crystal Si(1 0 0) and thermally oxidized Si(1 0 0) substrates by pulsed laser deposition were annealed in the temperature range of 700–1000 °C in air. The annealing temperature dependence of microstructure and magnetic properties of Ce:YIG films was studied using X-ray diffraction combined with vibrating sample magnetometer. The results show that single phase of polycrystalline Ce:YIG thin films can be obtained by the post-annealing of as-deposited films at the temperature of 700 °C. However, two steps of phase segregation of Ce:YIG occur as the post-annealing temperature increases: at first, Ce:YIG is decomposed into YIG and non-magnetic CeO2 when annealed at 800 °C; then YIG continues to be decomposed forming Fe2O3 when the temperature is increased up to 900 °C. Consequently, the saturation magnetization of Ce:YIG films decreases first and then increases with the post-annealing temperature going up, which indicates that the saturation magnetization of Ce:YIG films is mainly related to the phase composition of the films. Meanwhile, the presence of SiO2 buffer layer can significantly enhance the saturation magnetization of Ce:YIG films.

FeGeNi ternary alloy has been prepared by mechanical alloying followed by thermal treatment. Microstructure of the as-milled and annealed samples as well as thermally induced phase transition and variations in degree of order were investigated through a combination of X-ray diffraction and Mossbauer spectroscopy. The results show that alloying in the (Fe 0.81 Ni 0.19 ) 3.94 Ge 2 ternary system initiates during ball milling, with the appearance of partially D0 3 -ordered A2 solid solution phase and paramagnetic B8 2 FeGeNi ternary phase. A subsequent thermal treatment of 2 h at 800 K produces a remarkable increase in the degree of the D0 3 order, whilst ferromagnetic phase with amorphous character in the as-milled sample is transformed into paramagnetic B8 2 FeGeNi ternary phase where almost all Ni atoms occupy the vacant 2d site. Therefore in the annealed system coexist two phases, namely the D0 3 phase and paramagnetic B8 2 FeGeNi ternary phase.

Structural properties of Fe60Cr40 powders prepared by mechanical alloying in different atmosphere were investigated by using X-ray diffraction and 57Fe Mossbauer spectrum. The results show the formation of BCC paramagnetic structure when the sample sealed under Argon milled for 85 h. While in vacuum of 6×10−2 Pa, the BCC magnetic disordered solid solution is obtained for milled 45 h and more. The hyperfine filed distributions of Fe60Cr40 milled for 85 h in Argon is comprised of the crystalline grains and the disordered grain boundaries. Those of Fe60Cr40 milled for 85 h in vacuum indicate the formation of disordered alloys with nanoscale heterogeneous structures and the homogenization still proceed for a long milling time.

We have prepared FeSiBNbCu thin films by radio frequency (RF) magnetron sputtering and investigated the influence of the sputtering power on the microstructures of FeSiBNbCu thin films through analyzing their microstructural configurations by X-ray diffraction (XRD) and Mossbauer spectroscopy. The results showed that FeSiBNbCu thin films were amorphous when deposited with low sputtering power density and exhibited the mixed structure of crystalline and amorphous components with increasing the sputtering power density, without any heat treatment. The crystallites contained nanosized α-Fe(Si) and α-Fe(B) solid solutions of which the volume fractions and the microstructural configurations changed with the sputtering power.

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.

0.7Pb(Mg 1/3 Nb 2/3 )O 3 -0.3PbTiO 3 (PMNT) thin films were prepared on (111)Pt/Ti/SiO 2 /Si substrate by a modified sol-gel process with Nb 2 O 5 as the niobium source. XRD analysis shows that PMNT thin films with pure perovskite were obtained by spin-coating and annealing at 700°C for 20 minutes. The remanent polarization and coercive field of the PMNT thin films are about 7.69μC/cm 2 and 80.75kV/cm, respectively. The dielectric and C-V curve of PMNT thin films are also investigated. The dielectric constant ( e r ) reaches 935 and the dissipation factor (tanδ)is about 0.04 at 1kHz.