Zhipeng Gao

THE CONTRIBUTION OF ELECTRICAL CONDUCTIVITY, DIELECTRIC PERMITTIVITY AND DOMAIN SWITCHING IN FERROELECTRIC HYSTERESIS LOOPS

Triangular voltage waveform was employed to distinguish the contributions of dielectric permittivity, electric conductivity and domain switching in current-electric field curves. At the same time, it is shown how those contributions can affect the shape of the electric displacement — electric field loops (D–E loops). The effects of frequency, temperature and microstructure (point defects, grain size and texture) on the ferroelectric properties of several ferroelectric compositions is reported, including: BaTiO3; lead zirconate titanate (PZT); lead-free Na0.5K0.5NbO3; perovskite-like layer structured A2B2O7 with super high Curie point (Tc); Aurivillius phase ferroelectric Bi3.15Nd0.5Ti3O12; and multiferroic Bi0.89La0.05Tb0.06FeO3. This systematic study provides an instructive outline in the measurement of ferroelectric properties and the analysis and interpretation of experimental data.

Room temperature magnetoelectric coupling in intrinsic multiferroic Aurivillius phase textured ceramics

Spark plasma sintering was employed in order to obtain textured Aurivillius phase ceramics that simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature. The sintered multiferroics are layer-structured, nearly single-phase materials. Although a small amount of the secondary phase consisting of magnetic Co and Fe was detected by SEM/EDX, a majority of the observed ferromagnetic behaviour was attributed to the Aurivillius phase Bi4.25La0.75Ti3Fe0.5Co0.5O15 based on the observed magnetic anisotropy. The ferroelectric switching was demonstrated to exist in the Aurivillius phase ceramics by measuring the current peaks upon electric field reversal. Piezoresponse force microscopy at room temperature revelaed substantial changes of the ferroelectric domain structure when the Aurivillius phase material is subjected to an external magnetic field.

Bi3.25La0.75Ti2.5Nb0.25(Fe0.5Co0.5)0.25O12, a single phase room temperature multiferroic

Multiferroics (MFs) have attracted great research interest due to the coexistence of ferroelectric and magnetic ordering, as well as magnetoelectric (ME) coupling. At present, there is a very limited number of single-phase MFs known and these are still far from practical applications. In single-phase MFs, the simultaneous presence of electric and magnetic dipoles does not guarantee strong (ME) coupling, as the microscopic mechanisms of ferroelectricity and magnetism are quite different and do not intrinsically interact with each other. Here we show that in the ceramic system, Bi3.25La0.75Ti3−2xNbx(Fe0.5Co0.5)xO12, the x = 0.25 composition is ferroelectrically and ferromagnetically active at room temperature. A single-phase structure is supported by XRD, SEM/EDX and neutron diffraction data. Clear ME couplings were observed in this single-phase material at room temperature, where the magnetic iron and cobalt ions contribute to ferroelectric polarization and magnetic moment simultaneously. The results of structural, electrical and magnetic measurements are supported by first principle calculations. This material represents one of the first truly single phase bulk ceramics that exhibits multiferroic behaviour at room temperature and its discovery will help to guide the design of room temperature single-phase MFs with strong ME coupling for sensors and solid-state memory applications.

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

Abstract The solid solution system Nd2−xCexTi2O7 has been investigated. The solubility limit of Ce in Nd2−xCexTi2O7 was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd2−xCexTi2O7 (x = 0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point Tc of Nd2−xCexTi2O7 compounds. The results suggest that the Tc of Ce2Ti2O7 is <1445°C.