Yajun Qi

Sol-gel derived multiferroic BiFeO3 ceramics with large polarization and weak ferromagnetism

Insulating BiFeO3 ceramics of single perovskite phase were prepared by rapid sintering using sol-gel derived fine powders. The ceramics are dense and consist of grains of 2–6μm in diameter. Their leakage current density remains lower than 3.02×10−4A∕cm2 under the poling field below 119kV∕cm. The main conduction mechanism from 15to119kV∕cm is space-charge-limited current relating to oxygen vacancies. The ceramics exhibit a saturated ferroelectric hysteresis loop with a large remanent polarization (2Pr=56μC∕cm2) under the applied field of 180kV∕cm. Weak ferromagnetism with remanent magnetization of 1.5×10−5μB∕Fe is observed at 10K.

Large anisotropy of ferroelectric and dielectric properties for Bi3.15Nd0.85Ti3O12 thin films deposited on Pt∕Ti∕SiO2∕Si

Polycrystalline Bi3.15Nd0.85Ti3O12 (BNdT) thin films of a-axis preferential orientation [α(100)=62%] and high c-axis orientation [α(001)=96%] were fabricated directly on Pt∕Ti∕SiO2∕Si substrates through a sol-gel process. We observed strong dependences of ferroelectric and dielectric properties on the film orientation, with a remanent polarization 2Pr of 39μC∕cm2 and dielectric constant er of 343 at 100 kHz in the a-axis oriented film; 2Pr of 20μC∕cm2 and er of 331 in the film of random orientation; and 2Pr of 13μC∕cm2,er of 218 in the highly c-axis oriented film. Furthermore, the 2Pr value of a purely a-axis-oriented BNdT film can be predicted to be ∼51μC∕cm2. The large anisotropy of 2Pr and er values demonstrates that the polarization vector of BNdT is close to the a axis.

Ferroelectric and dielectric properties of Ca0.28Ba0.72Nb2O6 single crystals of tungsten bronzes structure

Large Ca0.28Ba0.72Nb2O6 (CBN-28) single crystals exhibit saturated ferroelectric hysteresis loops under an electric field of 115kV∕cm along the [001] direction. Their spontaneous polarization, remanent polarization and coercive field are 35.3μC∕cm2, 32.2μC∕cm2, and 38.1kV∕cm, respectively. The dielectric constant and dielectric loss tanδ of the crystals are 195 and 0.32 at 10 kHz, respectively. A diffused dielectric anomaly with relaxor characteristic was observed in the range of 325-500 °C at low frequencies, while the CBN-28 crystals experienced a first-order normal-relaxor ferroelectric phase transition around 252 °C on heating. A broad dielectric loss peak appears around 120 °C and it is interpreted in terms of the migration of oxygen vacancies. The conductance activation energy was determined to be 1.33 eV in the high temperature regime (500–560 °C).

Microstructural, ferroelectric, and dielectric properties of Bi3.15Nd0.85Ti3O12 ceramics

Dense Bi3.15Nd0.85Ti3O12 (BNdT) ceramics of a layered perovskite structure were sintered at 1100 °C by solid state reaction. Ferroelectric domains were probably observed in plate-shaped grains. The BNdT ceramics exhibit saturated ferroelectric hysteresis loops with a remanent polarization (2Pr) of 45μC∕cm2 and a coercive field of 67.6kV∕cm. The dielectric constant and dissipation factor of the ceramics at 100 kHz are 221 and 0.0064, respectively. A broad dielectric peak was observed around 408 °C, and it might come from the oxygen-vacancy-related dielectric relaxation. The leakage current density of the ceramics is less than 7.5×10−7A∕cm2 under an applied field below 239kV∕cm. The BNdT ceramics show Schottky emission behavior under low electric field below 75kV∕cm.

Giant magnetoelectric coefficient of Pb(Zr0.52Ti0.48)O3/La0.67Sr0.33MnO3 thin film grown on 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 single crystal assisted by metglas

Pb(Zr0.52Ti0.48)O3/La0.67Sr0.33MnO3 heterostructures grown on 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 single-crystal substrates with (001), (110), and (111) crystallographic orientations were fabricated. Their structural and ferroelectric properties were studied in detail, and their magnetoelectric properties assisted by metglas were also investigated. The results show that the maximum magnetoelectric coefficient of 17 V/(cmOe) can be achieved at a very low dc bias magnetic field of 14 Oe for the (001)-oriented heterostructure, which is related to the magnetic-mechanical-electric interaction between the magnetic and ferroelectric phases. This work provides a new route for realizing an extremely high magnetoelectric coefficient in a thin-film heterostructure at a low dc bias magnetic field.