Gao Jun-Xiong

Electric and Magnetic Properties of the (1?x)Ba0.6Sr0.4TiO3?xCoFe2O4 Multiferroic Composite Ceramics

The (1−x)Ba0.6Sr0.4TiO3−xCoFe2O4 multiferroic composite ceramics (x = 0.15−0.45) were fabricated by using the solid-state reaction method. X-ray diffraction shows that the composites are composed of the perovskite Ba0.6Sr0.4TiO3 phase and cubic spinel CoFe2O4 phase, without the other secondary phase. Scanning electron microscopy shows that the morphology of the composites is dense and comparatively homogenous. The variations of the dielectric constant and loss with frequency and temperature indicate that the composites behave as relaxor ferroelectrics. The composites display ferromagnetic and ferroelectric properties simultaneously. The saturated polarization of the composites decreases with increasing ferrite content, while the remnant polarization increases with the increasing ferrite content. The remnant polarization of the composite with x = 0.45 is even larger than that of the pure Ba0.6Sr0.4TiO3 ceramic. The enhanced ferroelectricity of the composites may be attributed to the space charge effect in the composites.

Magnetostatic Coupling in Ba0.8Sr0.2TiO3/CoFe2O4 Magnetoelectric Composite Thin Films of 2-2-Type Structure

Ba0.8Sr0.2TiO3/CoFe2O4(BST/CFO) magnetoelectric composite thin films of 2-2-type structures are prepared onto Pt/Ti/SiO2/Si substrates by a sol-gel process and spin coating technique. The structure of the prepared thin film is substrate/BST/CFO/.../CFO/BST. Three CFO ferromagnetic layers are separated from each other by a thin BST layer. The upper CFO layer is magnetostatically coupled with the lower CFO layer. Subsequent scanning electron microscopy investigations show that the prepared thin films exhibit good morphologies and have a compact structure, and the cross-sectional micrographs clearly display a multilayered nanostructure of multilayered thin films. The composite thin films exhibit good magnetic and ferroelectric properties. The spacing between ferromagnetic layers can be varied by adjusting the thickness of intermediate BST layer. It is found that the strength of magnetostatic coupling has a great impact on magnetoelectric properties of composite thin film; that is, the magnetoelectric voltage coefficient of the composite thin film tends to increase with the decrease of pacing between two neighboring CFO ferromagnetic layers as a result of magnetostatic coupling effect.

Effect of annealing on ferroelectric properties of Bi3.25La0.75Ti3O12 thin films prepared by the sol-gel method

Bi3.25La0.75Ti3O12(BLT) thin films were prepared on Pt/Ti/SiO2/Si substrate by the sol-gel method. The effect of annealing on their structures and ferroelectric properties was investigated. The XRD patterns indicate that the BLT films annealed at different temperatures are randomly orientated and the single perovskite phase is obtained at 550°C. The remnant polarization increases and the coercive field decreases with the annealing temperature increasing. The leakage current density of the BLT films annealed at 700°C is about 5.8×10−8 Al cm2 at the electric field of 250 kV/cm.

Ferroelectric Properties of Bi3.25La0.75 Ti3O12 Thin Films Crystallized in Different N2/O2 Ambients

Bi3.25La0.75 Ti3O12 (BLT) ferroelectric thin films are deposited by sol-gel method and annealed for crystallizaion in total 1sccm N2/O2 mixed gas with various ratio at 750°C for 30 min. The effect of crystallization ambient on the structural and ferroelectric properties of the BLT films is studied. The growth direction and grain size of BLT film are revealed to affect ferroelectric properties. After the BLT film is annealed in 20% O2, the largest Pr value is obtained, which is ascribed to an increase of random orientation and large grain size. The fatigue property is improved with the concentration of oxygen in the ambient increasing, which is ascribed to annealing in the ambient with high concentrated oxygen adequately decreasing the defects related to lack of oxygen.

EFFECT OF ANNEALING TEMPERATURE ON LEAKAGE CURRENT CHARACTERISTICS OF Bi3.25La0.75Ti3O12 THIN FILMS PREPARED BY SOL-GEL METHOD

ABSTRACT Bi3.25La0.75Ti3O12 films are prepared on Pt/Ti/SiO2/Si substrate by Sol-Gel method. The XRD patterns indicate that the BLT films annealed at different temperatures are randomly orientated and the single perovskite phase is obtained at 550°C annealed. The ferroelectric properties and leakage current characteristics are studied. The remnant polarization increases and the coercive field decreases with the annealing temperature increasing. The leakage current density of the BLT films annealed at 700°C is about 5.8 × 10−8 A/cm2 at 250 kV/cm. There are unambiguously differentiated mechanisms responsible for electrical transport at different electric field regimes in the BLT films.

Electric and Magnetic Properties and Magnetoelectric Effect of the Ba0.8Sr0.2TiO3/CoFe2O4 Heterostructure Film by Radio-Frequency Magnetron Sputtering

A Ba0.8Sr0.2TiO3/CoFe2O4 layered heterostructure film was grown on a Pt/TiO2/SiO2/Si substrate by rfmagnetron sputtering. X-ray diffraction shows that the film consists of perovskite Ba0.8Sr0.2TiO3 and spinel CoFe2O4 phases. The microstructures of the film were observed by a scanning electron microscope (SEM), showing good surface morphology and clear interfaces among the Ba0.8Sr0.2TiO3 film, the CoFe2O4 film and the substrate. The variations of dielectric properties with frequency of the heterostructure film are investigated. The heterostructure film simultaneously displays distinct ferroelectricity and ferromagnetism. Moreover, an obvious magnetoelectric coupling effect was observed in the heterostructure film with a maximum magnetoelectric voltage coefficient of 5.0 mV· cm−1 Oe−1, which is about seven times larger than that of the Ba0.8Sr0.2TiO3/CoFe2O4 particulate composite ceramics in a previous report.

Preparation and Characterization of BPO Film as Electrode for Using of FeRAM

Conductive perovskite BaPbO3 (BPO) films as a potential electrode material of PZT capacitors used in ferroelectric random access memory are prepared by rf magnetron sputtering. An x-ray diffractometer and standard four probe method are employed to investigate the dependence of growth conditions on crystal structure and conductivity of BPO films. It is found that BPO films with perovskite phase can be obtained at substrate temperatures above 425° C, and the sample with the lowest resistivity is obtained at 450° C under pure argon atmosphere. Using this BPO film as electrode, ferroelectric properties of BPO/PZT/BPO and Pt/PZT/BPO sandwiched structures are evaluated. Their remanent polarization and coercive field are 36.6μC/cm2 (81.3kV/cm) and 36.9μC/cm2 (89.1 kV/cm), respectively. The coercive field of the former structure is lower than that of the latter, but remanent polarizations are almost the same. In addition, the results imply that BPO electrode is helpful to improve the fatigue resistance of PZT. The reasons are discussed.

THE EFFECT OF SEEDING LAYERS ON FERROELECTRIC PROPERTIES OF PTZT THIN FILMS

ABSTRACT In this paper, the TiOx and PbO seeding layers have been used to study the influence of Pb1.08(Ta0.005Zr0.3Ti0.695)O3 (PTZT) thin films on a stabilized Pt/Ti/SiO2/Si bottom electrode. The PTZT and PbO/PTZT/PbO XRD measurements show that the major perovskite orientations (100), (110) and (111) have been found together with pyrochlore and the TiOx/PTZT/TiOx XRD measurement displays strong (111)-preferred orientation. The best ferroelectrics values and fatigue properties have been measured for the sample TiOx/PTZT/TiOx film. And strongly asymmetric polarization hysteresis loops are observed in the sample PbO/PTZT/PbO. We take the view that the origin of the hysteresis loop offsets and asymmetry is caused by a strong electric field within a thin surface layer in which the ferroelectric polarization is smaller or even absent compared to the bulk of the film. The fatigue characteristic of PTZT thin films suggests that the TiOx and PbO seeding layers alleviate the accumulation of oxygen vacancies near the interface which has been proposed as a primary cause of fatigue in lead base perovskite thin films.

Study on the I–V characteristics of ferroelectric thin film systems with the structure of MFSM

Abstract Ferroelectric thin film systems with multilayer structure, Au/PZT/p-Si, and Au/PZT/BIT/p-Si, were fabricated by using pulsed laser deposition (PLD) technique. Ferroelectric PZT and BIT layers with the thickness of 400 and 100 nm, respectively, were grown on (100) oriented p-type silicon substrates. Electrical properties of the metal/ferroelectric/semiconductor/metal (MFSM) structures were characterized through the measurements of bias voltage dependence of current. The conductivity behavior is discussed. The results suggest that the growth of the BIT buffer layer has decreased the serious interaction and interdiffusion in the PZT/p-Si interface. The I–V hysteresis loop is large enough to identify the current of reading ‘1’ and ‘0’, respectively, indicating that the access function is realized in our ferroelectric thin film systems.

The domain structure and polarization retention properties of PT/PZT/PT ferroelectric thin film

The highly oriented perovskite-phase PT/PZT/PT ferroelectric thin film was prepared by sol-gel method. The domain structures and polarization retention properties were investigated by scanning force microscopy. The amplitude and phase images of piezoresponse show complex various contrasts of dark, bright and gray. The complex variation of contrast in piezoresponse images results from the perplexing orientation of grains and arrangement of domains in the ferroelectric films. The bright and dark areas in phase images correspond to top-to-bottom and bottom-to-top polarization oriented c-domain, respectively. The gray areas are c-domains with the polarization vector deviating from the direction normal to the film plane. The surface potential images of EFM are bright contrast, which is due to positive charges trapped on the film surface after being polarized by positive voltage. And the brighter contrast is obtained from the higher electric field. The time-dependent surface potential images and line potential profiles show that the potential decays with time. And the decay in the region polarized by higher electric field is faster, especially at 15 min. This indicates that the polarization retention is related to the polarized electric field. Better retention properties may be obtained from a proper polarized electric field.