Xiaoding Qi

Understanding the formation of ultrafine spinel CoFe2O4 nanoplatelets and their magnetic properties

Cobalt ferrite (CoFe2O4, CFO) nanoplatelets with a rhomboidal shape were synthesized through a facile hydrothermal route using a very low concentration precursor at low temperatures. The effects of reaction temperature and time on the morphologies as well as the sizes of the products were systemically investigated. The as-synthesized CFO nanocrystals showed a special rhomboidal shape with crystal growth along the (111) direction. The very low concentration precursor plays an important role in forming CFO nanocrystals with such special shape and ultrafine size. The single-phase CFO nanoplatelets synthesized at 180 °C with size of 17 nm present high saturation magnetization (79.7 emu/g) and high coercivity (3100 Oe). The preparation conditions have significant effects on the crystal size and shape, magnetization and relaxation activation of the CFO nanoplatelets.

Dielectric relaxation and magnetic behavior of bismuth-substituted yttrium iron garnet

Y3−xBixFe5O12 (x = 0–1.0) was prepared by solid state reaction. X-ray diffraction showed a gradual increase of lattice parameter with the Bi concentration, indicating that Bi had been incorporated into the lattice. Dielectric responses of the samples at room temperature increased initially at low Bi (x < 0.4) and then decreased rapidly at high Bi (x ≥ 0.6). Temperature dependence of dielectric constant showed a huge steplike increase at the temperatures between 20 and 200 °C, increasing with Bi. X-ray photoelectron spectroscopy revealed the presence of Fe2+ and Bi2+ in the samples, which might be responsible for the dielectric behaviors. An increase of coercivity and Curie temperature with Bi was observed. This was accompanied by a reduction of the saturation magnetization, whose variation with T follows a modified Bloch law.

Ferroelectric properties and dielectric responses of multiferroic BiFeO3 films grown by RF magnetron sputtering

Multiferroic BiFeO3 films have been grown on LaNiO3−x/SrTiO3 and Pt/Si substrates by RF magnetron sputtering. The films showed fully saturated ferroelectric hysteresis loops with large remanent polarization of 64 µC cm−2, suitable for most device applications. Piezoresponse force microscopy confirmed that the films were electrically writable. In addition to the high-frequency intrinsic dielectric loss of epitaxial films, the Argand diagram also revealed low-frequency contributions from both dc conductivity and interfacial polarization at electrodes. For polycrystalline films on Pt/Si, the dominant contribution to dielectric loss was space charge polarization at grain boundaries.

Reduced room-temperature ferromagnetism in intermediate conducting regime of v doped ZnO

The mechanism of room temperature (RT) ferromagnetism for low doping concentration of 2.5% V in ZnO have been systematically discussed by measuring structural, electrical, and magnetic properties. The evolution of the Curie temperature from above RT in insulated V:ZnO powders to 270 K in semiconducting samples was observed with increasing carrier concentration by using different hydrogenated annealing treatment. The results provide a direct observation that the free carriers suppress the ferromagnetism and might be associated the phenomena of charge-transfer and interaction between bound magnetic polarons.

Exchange bias in bilayers based on the ferroelectric antiferromagnet BiFeO3

This paper reports studies of the exchange interaction between the conventional ferromagnets CoFe2O4 and permalloy and the multiferroic antiferromagnet BiFeO3 films. Bilayer films were grown which demonstrated significant exchange bias at room temperature. BiFeO3 films grown under the same conditions also showed a large electric polarization. The results are encouraging for room-temperature applications of BiFeO3 with the combined properties of antiferromagnetism and ferroelectricity, but the significant residual leakage in our BiFeO3 films prevented the demonstration of a stable, controllable exchange bias.

Structural transformation and charge transfer induced ferroelectricity and magnetism in annealed YMnO3

Multiferroic materials such as YMnO3, which uniquely exhibit ferroelectricity and magnetism simultaneously, have been extensively studied for spintronic device applications. However, the origin of multiferroicity remains poorly understood. In this study, the structural phases of YMnO3 ceramics and their lattice distortions after careful annealing were investigated to explain the origins of their multiferroicity. A structural transition from the orthorhombic to the hexagonal phase was observed when the annealing temperature reached around 1100 °C. This structural transformation also results in a magnetic transition from 3D Mn-O-Mn to 2D Mn-O-Mn superexchange coupling. The ferroelectricity was enhanced by escalation of the structural distortion caused by the rising annealing temperature. The annealing effect also results in the re-hybridization of the electronic structure of YMnO3. X-rayabsorption near-edge spectra suggest that there is charge transfer from the Y-OT (apical oxygen) bonds of Y 4d-O 2p hybridized states to the OT-Mn bonds of Mn 3d-O 2p hybridized states, which is responsible for the enhanced ferroelectricity. This approach could be used to probe the origin of the ferroelectricity and multiferroic properties in rare-earth manganites.

In-situ post-annealing technique for improving piezoelectricity and ferroelectricity of Li-doped ZnO thin films prepared by radio frequency magnetron sputtering system

Li-doped zinc oxide (L0.03Z0.97O) thin films are deposited onto Pt/Ti/SiO2/Si substrates via the radio frequency magnetron sputtering method. The structure evolution with annealing temperature of the predominantly (002)-oriented Li-doped ZnO (LZO) films after in-situ post-annealing process is determined. The largest values of the piezoelectric coefficient (d33) and the remnant polarization (Pr) (22.85 pm/V and 0.655 μC/cm2, respectively) are obtained for LZO films post-annealed at 600 °C, which can be attributed to the predominant (002)-oriented crystalline structure, the release of intrinsic residual compressive stress, and less non-lattice oxygen.

Dielectric and magnetic properties of Y3−xTbxFe5O12 ferrimagnets

Y3−xTbxFe5O12 (x = 0−3) were prepared by the solid state reaction at 1350 °C. X-ray diffraction confirmed the obtained samples of a pure garnet phase. The dielectric responses were investigated over a wide frequency and temperature range. Two large, step-like increases of dielectric constants were observed at 20–100 °C and 170–270 °C, respectively. Thermal activation of the polarons related to Fe2+ was responsible for the observed dielectric behaviors. The Fe2+/Fe3+ hopping process was faster in Tb3Fe5O12 than in Y3Fe5O12. The saturation magnetization versus temperature measurements revealed the existence of a compensation point for all Y3−xTbxFe5O12 with x > 0, owing to the cancellation of net Fe3+ moments by Tb3+. The compensation temperature fell from 256 K at x = 3 to below 77 K at x = 1, because the reduced moment by the Tb3+ number reduction had to be supplemented by the increased Tb3+ interaction at lower temperature.

Electrical transport and ac conductivity properties of hydrogenated annealing V-doped ZnO

The hydrogenated annealing effects on structure, magnetism, electrical transport, and ac conductivity for V-doped ZnO powders have been systematically investigated. Room temperature ferromagnetism has been observed for the hydrogenated V:ZnO powders. The saturation magnetization increases with hydrogenated annealing temperature (Tha). By the analysis of electrical transport and ac conductivity, the V:ZnO powders show Efros’s variable range hopping and the density of microstructural defects increases with Tha. The results suggest that the fluctuation of the magnetization is strongly correlated with the defect density in V:ZnO powders.

Single Crystal Growth and Characterisations of Transition Metal Ion Doped TTB Ferroelectrics

Tetragonal tungsten-bronze (TTB) structured niobates were doped with transition metal (TM) ions in order to search for multiferroic materials. Single crystals of Ba6FeNb9O30, Ba6CoNb9O30, Ba6Mn0.67Nb9.33O30, K6Fe3.0Nb9.6O30, etc. were grown by spontaneous nucleation method and their structures were determined by X-ray single crystal diffraction, which revealed that TM substituted with Nb instead of entering any of the three interstitial sites. Polarisation vs. electrical field measurements showed hysteresis loops for Ba6FeNb9O30, suggesting that it is likely to be ferroelectric just as many other TTB niobates. Magnetic measurements indicated some ordered magnetic structure at low temperature.