Huifeng Bo

Oxygen-vacancy-related high-temperature dielectric relaxation in SrTiO3 ceramics

Quantum paraelectric SrTiO3 has resulted in many investigations because of the anomalous properties. Here, using the conventional solid-state reaction method, we fabricated polycrystalline SrTiO3 ceramics with pure cubic perovskite structure. A dielectric loss peak is observed at around 450 K and 100 Hz and it shifts to higher temperature with increasing frequency. The typical high-temperature dielectric relaxation process is confirmed to be related to the oxygen vacancies (OVs) inside ceramics. More interestingly, a Cole–Cole fitting to loss peaks reveals a weaker correlation among OVs for such dielectric materials compared with that of ferroelectrics.

Critical radii of ferroelectric domains for different decay processes in LiNbO3 crystals

The LiNbO3 crystals were polarized at fixed points and characterized by means of scanning probe microscope. The growth evolution and the decay process of the fabricated domains were carefully studied. Two critical radii rC and Rc that determine a different decay behavior of domains were put forward, where rC is considered to correspond to the point when the domains just penetrate the crystal. Those domains with radius larger than rC could exist beyond 5days, those between rC and Rc experienced a distinct metastable state before completely disappearing, while those smaller than Rc were even invisible under piezoresponse force mode.

Studying the Polarization Switching in Polycrystalline BiFeO3 Films by 2D Piezoresponse Force Microscopy.

For rhombohedral multiferroelectrics, non-180° ferroelectric domain switching may induce ferroelastic and/or (anti-)ferromagnetic effect. So the determination and control of ferroelectric domain switching angles is crucial for nonvolatile information storage and exchange-coupled magnetoelectric devices. We try to study the intrinsic characters of polarization switching in BiFeO3 by introducing a special data processing method to determine the switching angle from 2D PFM (Piezoresponse Force Microscopy) images of randomly oriented samples. The response surface of BiFeO3 is first plotted using the piezoelectric tensor got from first principles calculations. Then from the normalized 2D PFM signals before and after switching, the switching angles of randomly oriented BiFeO3 grains can be determined through numerical calculations. In the polycrystalline BiFeO3 films, up to 34% of all switched area is that with original out-of-plane (OP) polarization parallel to the poling field. 71° polarization switching is more favorable, with the area percentages of 71°, 109° and 180° domain switching being about 42%, 29% and 29%, respectively. Our analysis further reveals that IP stress and charge migration have comparable effect on switching, and they are sensitive to the geometric arrangements. This work helps exploring a route to control polarization switching in BiFeO3, so as to realize desirable magnetoelectric coupling.

Multiferroic Properties of CoFe2O4/PbZr0-52Ti0.48O3 Composite Ceramics

The xCoFe 2 O 4 -(1−x)PbZr 0.52 Ti 0.48 O 3 (PZT) composite ceramics with x = 0.1, 0.2 and 0.3 have been prepared by solid state reaction method. X-ray diffraction result reveals that no chemical reaction or phase diffusion between the CoFe 2 O 4 and PZT phases exists in the composites. The composites exhibited ferroelectric as well as magnetic characteristics at room temperature. The distribution of the ferroelectric and magnetic domains has been observed by Scanning Probe Microscopy (SPM). The dielectric properties of the composite ceramics were also investigated. The dependence of dielectric properties and the morphology of both the ferroelectric and magnetic domains on CoFe 2 O 4 concentration were discussed in detail.

Optical properties of Bi3.15Nd0.85Ti3O12 nanostructures

Ferroelectric materials are widely researched because of their application in electronic devices. A ferroelectric nanotube is important both theoretically and experimentally because of its nonplanar structure. Bi3.15Nd0.85Ti3O12 nanotubes are fabricated utilizing templates of porous anodic aluminum oxide and thin films are synthesized as well for comparison. Atomic force microscopy, scanning electron microscopy, and x-ray diffraction reveal the microstructure of the samples. The optical properties are investigated carefully to reveal the size effect with the decrease in sample dimensionality. The observed shift of the absorption edge for different nanostructures is discussed.

Growth evolution and decay properties of the abnormally switched domains in LiNbO3 crystals

Direct domain writing is carried out in single-crystalline stoichiometric LiNbO3 crystals using a scanning probe microscope. The abnormally switched domains with polarization antiparallel to the poling field are observed, and their growth and decay processes are systematically studied. The radius of these domains is proportional to the pulse magnitude while it remains constant as the pulse width varies. These abnormal domains quickly decay, the lifetime of which is linear with the poling pulse magnitude while it stretched exponentially increases with the increasing pulse width. The experimental results are carefully discussed in relation to the reported charge injection model.

Decay properties of artificial two-domain structures in LiNbO3 crystals studied by scanning probe microscope

Decay processes of artificial two-domain structures with various distances in LiNbO3 crystals were carefully investigated using scanning probe microscope. The decay behavior of the composite domains is found closely related to the set domain distance. Detailed analysis indicates that the tip field mostly affects the lateral domain wall motion of the domains in two-domain structures, while the repulsive force of bound charges mainly influences vertical domain wall motion. Thin sample thickness and large domain distance are inferred beneficial for manufacturing small scaled stable domain structures.

Drive frequency dependent phase imaging in piezoresponse force microscopy

The drive frequency dependent piezoresponse (PR) phase signal in near-stoichiometric lithium niobate crystals is studied by piezoresponse force microscopy. It is clearly shown that the local and nonlocal electrostatic forces have a great contribution to the PR phase signal. The significant PR phase difference of the antiparallel domains are observed at the contact resonances, which is related to the electrostatic dominated electromechanical interactions of the cantilever and tip-sample system. Moreover, the modulation voltage induced frequency shift at higher eigenmodes could be attributed to the change of indention force depending on the modulation amplitude with a piezoelectric origin. The PR phase of the silicon wafer is also measured for comparison. It is certificated that the electrostatic interactions are universal in voltage modulated scanning probe microscopy and could be extended to other phase imaging techniques.

Influence of feedback parameters on ferroelectric domain imaging with piezoresponse force microscopy

The piezoresponse (PR) amplitude and phase signals for antiparallel ferroelectric domains were investigated with different feedback parameters. It is found that the drive frequency and setpoint can strongly alternate the PR image contrast, while the drive phase can be considered as a constant adding to the PR phase signal. The PR amplitude is proportional to the drive amplitude while the PR phase is drive amplitude independent. The larger piezoelectric vibration amplitude and fitting piezoelectric constants (PCs) obtained by vectorial analysis compared with the known values are originated from the sample resonance, and the local electrostatic force can lead to a nonlinear shift in the measured PCs from the theoretical expectations.

Ferroelectric and dielectric properties of Bi3.15Nd0.85Ti3O12 nanotubes

In order to match the high-density requirement of ferroelectric memories, ferroelectric Bi3.15Nd0.85Ti3O12 nanotubes with outer diameter of about 100 nm and wall thickness of about 30 nm were synthesized using a sol-gel method. Transmission electron microscope images and Raman spectra revealed the Bi-layered perovskite structure of these nanotubes. Their dielectric constant and remnant polarization were comparable with those of thin film form. Piezoelectric hysteresis loops of individual nanotube measured by piezoresponse force microscope indicate their asymmetry, and the switched nanotubes show long term retention.