X. H. Wang

Giant piezoelectric d33 coefficient in ferroelectric vanadium doped ZnO films

A giant electromechanical d33 coefficient 110pC∕N is obtained in ferroelectric V-doped ZnO films, which is nearly one order of magnitude higher than that of undoped samples. It is considered that the switchable spontaneous polarization induced by V dopants and the accompanying relatively high permittivity should be responsible for the enhancement of piezoelectric response. Moreover, from another point of view, an easier rotation of V–O bonds which are noncollinear with c axis under electric field might be the microscopic origin of this anomaly. The improved piezoelectric properties could make V-doped ZnO a promising candidate for piezoelectric devices.

Manifestation of ferroelectromagnetism in multiferroic BiMnO3

Multiferroic BiMnO3 with a highly distorted perovskite structure induced by the stereochemically active 6s2 electron lone pairs of Bi3+ was synthesized at a high pressure of 6 GPa. Magnetization, differential scanning calorimetry, dielectric permittivity, and in situ powder x-ray diffraction as a function of temperature were carried out, respectively. In light of comprehensive evaluation, we can conclude that the synthetic BiMnO3 ceramic displays ferromagnetic and ferroelectric orderings simultaneously, i.e., ferroelectromagnetism below its ferromagnetic Curie temperature TM∼100K.

Cr-substitution-induced ferroelectric and improved piezoelectric properties of Zn1−xCrxO films

(0001) oriented polycrystalline Cr-doped ZnO films have been prepared on n-Si(111) single-crystal substrates by nonequilibrium reactive magnetron cosputtering. The c-axis texture of the films weakens and a transformation of doping mechanism from CrZn to CrZn+Cri is indicated as the doping concentration increases. The Cr dopants are demonstrated to exist as Cr3+ ions in the films. Ferroelectric measurements show that the Ag∕Zn0.94Cr0.06O∕n-Si heterostructure displays well-defined hysteresis loop with a remanent polarization ∼0.2μC∕cm2 and a coercive field ∼50kV∕cm at room temperature. The capacitance-voltage curves with clockwise traces show typical memory windows, which symmetrically widen as the sweep amplitude increases. Ferroelectricity in Cr-doped ZnO was also established by a displacement-voltage “butterfly” loop. The observed ferroelectric behavior is attributed to the partial replacement of host Zn2+ ions by smaller Cr3+ ions, which occupy off-center positions and thereby induce permanent electric ...

Room temperature multiferroic behavior of Cr-doped ZnO films

Single-phase 9 at. % Cr-doped ZnO film has been prepared on Pt(111)/Ti/SiO2/Si(100) substrates by reactive sputtering method. The film is found to present ferroelectric and ferromagnetic behaviors simultaneously at room temperature, and it undergoes transitions to paraelectric and paramagnetic phases at ∼368–373 and ∼495 K, respectively. It is considered that the local electric dipoles induced by the distortions of CrO4 tetrahedra should be responsible for the ferroelectricity. On the other hand, the ferromagnetic ordering could be explained by the interaction of the localized spins with statically occupied polaron states. The multiferroic behavior adds a dimension to the multifunction of ZnO.

Ferroelectricity of multiferroic hexagonal TmMnO3 ceramics synthesized under high pressure

Dense hexagonal TmMnO3 ceramics were synthesized by solid-state reaction technique combined with high-pressure treatment which significantly increased the density of ceramic samples. The crystal structure of the hexagonal TmMnO3 oxide was refined by using Rietveld analysis based on powder x-ray diffraction experiment. We observed obvious dielectric peaks through dielectric measurement on the specimen subjected to postannealing in oxygen atmosphere. A ferroelectric-paraelectric transition around 348°C is identified. Polarization-electric field hysteresis (P-E) loop measurement proved the ferroelectricity of the sample at room temperature.

Photoluminescence and photoabsorption blueshift of nanostructured ZnO: Skin-depth quantum trapping and electron-phonon coupling

Although the size- and shape-induced blueshift in the photoluminescence and photoabsorption of nanostructured ZnO has been extensively investigated, the underlying mechanism remains yet unclear. Here we show that theoretical reproduction of the observed trends clarifies that the blueshift originates from the Hamiltonian perturbation due to the broken-bond-induced local strain and quantum trapping and electron-phonon coupling in the surface skin up to two atomic layers in depth while bonds in the core interior retain their bulk nature. The extent of the blue shift depends on the tunable fraction of undercoordinated atoms in the surface skin. Therefore, the quantum confinement effect is indeed more “superficial” than first thought [H. Winn, OE Mag. 8, 10 (2005)].

Large magneto (thermo) dielectric effect in multiferroic orthorhombic LuMnO3

We have investigated the relation between ferroelectric and magnetic orders of orthorhombic (o-) LuMnO3 ceramics. The increase of dielectric constant ɛ exceeds 82% near incommensurate to commensurate E-type antiferromagnetic (AFM) spin ordering transition temperature TL, reflecting a large magneto (thermo) dielectric response. Meanwhile, distinct anomalies and thermal hysteresis behavior are observed near this temperature in both temperature dependence of ɛ and specific heat Cp, indicating a strong coupling between FE and magnetic orders in o-LuMnO3. Comparing to o-HoMnO3, TmMnO3, and YbMnO3 with similar E-type AFM ground state, o-LuMnO3 has the largest magneto (thermo) dielectric effect

Fabrication and properties of epitaxial growth BiScO3–PbTiO3 thin film via a hydrothermal method

BiScO3–PbTiO3 thin film was hydrothermally deposited on Nb-doped SrTiO3 [100] single crystal substrate at 180°C with a large thickness of 3.1μm. The x-ray diffraction and high resolution transmission electron microscopy confirmed the epitaxial growth relationship between the BSPT thin film and Nb-STO substrate. Single-crystal-like polarization hysteresis loops were observed, with the remanent polarization of 30μC∕cm2. Piezoelectric force microscopy revealed a single +c domain structure of the film, while exceptional linear piezoelectric response was observed with the effective piezoelectric constant d33* of 65pm∕V. The temperature dependence of the effective d33* indicated that this film exhibited a high temperature stability.

Pressure-induced phase transition in Ho0.8Dy0.2MnO3 multiferroic compound

The multiferroic Ho0.8Dy0.2MnO3 compound crystallizing into hexagonal perovskite was investigated by using an in situ high pressure Raman scattering method at room temperature. It is found that with increasing pressure, the compound started to transform from hexagonal-type into an orthorhombic-type perovskite near 9.8GPa. The phase transition process is analyzed.

The effects of high pressure on the ferroelectric properties of nano-BaTiO3 ceramics

Temperature dependence of the dielectric constant of nano-grain BaTiO3 (BTO) ceramic has been investigated under hydrostatic pressure up to 5 GPa. We show that the paraelectric-ferroelectric phase transition temperature, which is represented by TC (Curie temperature point), decreases with pressure at a rate of dTC/dσ = -40.0 K/GPa for coarse grain ceramic and -34.3 K/GPa for ceramic with 60nm size grains. For the paraelectric-ferroelectric phase transition temperature, the variation of inter-granular stress with reducing grain size is considered the main factor causing the decrease of TC and the change of dTC/dσ. Adding the hydrostatic-pressure and inter-granular stress contributions to a Ginsburg-Landau-Devonshire (GLD) type thermodynamic theory provides a satisfactory explanation to our experimental results. Based on the GLD theory, a phase diagram, reflecting the relationship among the transition temperature, grain size and hydrostatic-pressure, has been established, which matches well with our experimental results.