Ji Zhang

Synthesis, structures and properties of single phase BiFeO3 and Bi2Fe4O9 powders by hydrothermal method

Single phase BiFeO3 and Bi2Fe4O9 powders have been synthesized via hydrothermal method by carefully controlling the reaction conditions. It is found that the BiFeO3 shows dense microstructure with irregular, larger grains, the average grain size is ~5.0xa0μm, whereas the Bi2Fe4O9 displays small, porous microstructure with the sheet shaped grains, the average thickness is ~55xa0nm and length is ~500xa0nm. X-ray photoemission spectra confirm that in both compositions, the Bi and Fe cations have the charge valence of +3. The magnetization–magnetic field (M–H) and magnetization–temperature (M–T) measurements reveal that the BiFeO3 has weak ferromagnetism even at room temperature due to its canted antiferromagnetic interaction, whereas the Bi2Fe4O9 shows antiferromagnetic nature in the temperature range of 10–300xa0K. Our results provide interesting supplements for controlling the morphology and understanding the structure–property relationship of BiFeO3 and Bi2Fe4O9.

Giant positive magnetoresistance in half-metallic double-perovskite Sr2CrWO6 thin films

Ferrimagnetic half-metallic Sr2CrWO6 thin films show giant positive magnetoresistance up to 17,200%. Magnetoresistance (MR) is the magnetic field–induced change of electrical resistance. The MR effect not only has wide applications in hard drivers and sensors but also is a long-standing scientific issue for complex interactions. Ferromagnetic/ferrimagnetic oxides generally show negative MR due to the magnetic field–induced spin order. We report the unusually giant positive MR up to 17,200% (at 2 K and 7 T) in 12-nm Sr2CrWO6 thin films, which show metallic behavior with high carrier density of up to 2.26 × 1028 m−3 and high mobility of 5.66 × 104 cm2 V−1 s−1. The possible mechanism is that the external magnetic field suppresses the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations, which enhance electronic scattering and lead to the giant positive MR. The high mobility may also have contributions to the positive MR. These results not only experimentally confirm that the giant positive MR can be realized in oxides but also open up new opportunities for developing and understanding the giant positive MR in oxides.

Room-Temperature Multiferroics and Thermal Conductivity of 0.85BiFe1–2xTixMgxO3–0.15CaTiO3 Epitaxial Thin Films (x = 0.1 and 0.2)

Thin films of 0.85BiFe1-2xTixMgxO3-0.15CaTiO3 (x = 0.1 and 0.2, abbreviated to C-1 and C-2, respectively) have been fabricated on (001) SrTiO3 substrate with and without a conductive La0.7Sr0.3MnO3 buffer layer. The X-ray θ-2θ and ϕ scans, atomic force microscopy, and cross-sectional transmission electron microscopy confirm the (001) epitaxial nature of the thin films with very high growth quality. Both the C-1 and C-2 thin films show well-shaped magnetization-magnetic field hysteresis at room temperature, with enhanced switchable magnetization values of 145.3 and 42.5 emu/cm3, respectively. The polarization-electric loops and piezoresponse force microscopy measurements confirm the room-temperature ferroelectric nature of both films. However, the C-1 films illustrate a relatively weak ferroelectric behavior and the poled states are easy to relax, whereas the C-2 films show a relatively better ferroelectric behavior with stable poled states. More interestingly, the room-temperature thermal conductivity of C-1 and C-2 films are measured to be 1.10 and 0.77 W/(m·K), respectively. These self-consistent multiferroic properties and thermal conductivities are discussed by considering the composition-dependent content and migration of Fe-induced electrons and/or charged point defects. This study not only provides multifunctional materials with excellent room-temperature magnetic, ferroelectric, and thermal conductivity properties but may also stimulate further work to develop BiFeO3-based materials with unusual multifunctional properties.

Simultaneously enhanced ferroelectric and magnetic properties in 0.675BiFe1−xCrxO3–0.325PbTiO3 (x = 0–0.05) ceramics

Abstract Multiferroic ceramics of 0.675BiFe1−xCrxO3–0.325PbTiO3 (xxa0=xa00, 0.01, 0.025, 0.05) were prepared and comparatively investigated. The rhombohedral–tetragonal morphotropic phase boundary (MPB) was formed in 0.675BiFeO3–0.325PbTiO3. The B-site Cr-substitution changes the structure from MPB to rhombohedral phase gradually, accompanied with monotonously decreased ferroelectric Curie temperature and average grain size. More interestingly, Such Cr-substitution enhances room temperature ferroelectric and magnetic properties simultaneously. The underlying mechanism is mainly attributed to the substitution enhanced local Fe3+–Cr3+ magnetic interaction and the suppressed oxygen vacancy effect. We believe these results are helpful supplements for optimizing structures and room temperature multiferroic properties of BiFeO3-based materials.