Zhenping Chen

Magnetization reversal and Yb3+/Cr3+ spin ordering at low temperature for perovskite YbCrO3 chromites

The temperature dependence of the dc magnetization and the specific heat capacity are systematically investigated for the perovskite YbCrO3 chromites. The results show that there exist two complex sequences of magnetic transitions with the characteristics of magnetization reversal and Yb3+/Cr3+ spin ordering at different temperature, respectively. The antiferromagneticlike transition around TN=118 K is attributed to the antiferromagnetic ordering of the Cr3+ spins and a negative magnetization, accompanied with a tendency to the plateau below 10 K caused by the Yb3+ ordering. We obtain the Weiss constant of −197.0 K and an effective moment of 5.99μB for the sum of the free ion values of 4.53μB for Yb3+ and 3.87μB for high-spin Cr3+ from the experiments, which shows an antiferromagnetic interaction and the existence of weak canted antiferromagnetic (CAFM) characteristics. Corresponding to the magnetic phase transition in the Cr sublattice, the specific heat capacity under zero fields exhibits a sharp λ-shap...

Dependence of positron lifetime parameters on the preparation techniques and structural defects for YBCO cuprates

Based on possible existence of technique effect and scattered distribution in positron results, a special investigation was made for YBa2Cu3O7 � d systems in this paper. The dependence of preparation techniques and the structural characteristics on the sintering temperature and time are also given by positron lifetime, XRD and SEM experiments. The results show that the positron lifetime parameters and the microstructure show a significant dependence on sintering temperature and time. It is proved that, in the range of 920–950 jC/12–72 h, the positron results have good stability and reliability in the sintering times and temperature. The distribution characteristic of defect was evaluated and had a good stability in the preparation technique. The present studies provide an important experimental evidence for the study of Y-123 systems by positron experiment. D 2002 Elsevier Science B.V. All rights reserved.

Effect of calcining temperature on the microstructure and magnetic properties of CuFeO2 multiferroic ceramic

ABSTRACT Multiferroic ceramic CuFeO2 has been successfully fabricated by traditional solid-state reaction method under different calcining temperatures. X-ray powder diffraction results show that a higher calcining temperature is beneficial to form single delafossite phase with a hexahedron crystal structure. Raman and scanning electron microscopy results indicate that the proper calcining temperature is favourable to enhance the polarisation strength, grain size and density of samples. Magnetic properties results reveal that all samples exhibit two successive magnetic transitions with decreasing temperature, the corresponding magnetic transition temperature TN1 and TN2 are independent on calcining temperature. It is also found that the magnetic state of all samples at T = 10 and 13 K is weak ferromagnetic or ferrimagnetic state but not the antiferromagnetic state in theory. The value of magnetic susceptibility and saturation magnetisation are effectively enhanced by changing calcining temperature. The relationship between the microstructure and magnetic properties of CuFeO2 system is discussed in this paper.

Microstructure and Dielectric Properties of SnO2-doped CaCu3Ti4O12 Ceramics

Dielectric CaCu3Ti4O12-xSnO2 ceramics (x = 0, 0.003, 0.006, 0.010, 0.015 and 0.020) have been synthesized by solid-state reaction method. The microstructure was studied by X-ray diffraction, Raman spectrum and Scanning electron microscope. It is revealed that SnO2-doping results in the distortion of crystal structure and various grain growths for CaCu3Ti4O12-based ceramics. Dielectric measurement shows that lower-doped samples (x = 0.003 and 0.006) exhibit enhanced dielectric constants which are over 70,776 and 26,406 at the range of 40-104 Hz, respectively. In addition, the dielectric properties are explained in terms of internal barrier layer capacitance model by using the impedance spectroscopy analysis.

Hole carrier localization and Pr substitution in YBCO systems by positron experiment

Abstract Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0 to 1.0. The present results show that the value of short lifetime τ 1 in PrBa 2 Cu 3 O 7− δ is nearly the same as in the isostructural YBa 2 Cu 3 O 7− δ and the defect-related positron lifetime component τ 2 decreases as a function of Pr-substitution x . The increase of local electron density n e reflects the electronic intense localization. These experimental results are interpreted in terms of the localization of hole carriers in the Cu–O chains on Pr substitution due to disorder in the Ba and Pr planes by the partly occupation of Ba 2+ ion site with Pr 3+ ion. These will destroy the intrinsic conductivity of the Cu–O chains and so impresses the superconductivity above x =0.6 Pr-substitution concentration.

The structure, defects, electrical and magnetic properties of BiFe 1− x Zr x O 3 multiferroic ceramics

BiFe1−xZrxO3 (x = 0.00–0.30) ceramics were synthesized using solid state reaction method followed by rapid liquid phase sintering, and the microstructure, electrical and magnetic properties of the synthesized ceramics were systematically investigated. The XRD patterns show that no impurity phases exist in Zr doped samples; Zr doping induces the crystal structure distortion when x ≤ 0.10, and a structural phase transition occurs when the content of Zr varies from 0.10 to 0.20. SEM observations indicate that the average grain size is remarkably decreased by Zr doping. Positron annihilation lifetime spectra results indicate that cation vacancy-type defects exist in all samples, the cation vacancy concentration increases with increasing Zr content from 0.00 to 0.20, and then decreases with further increase of Zr content. Electrical and magnetic measurements show that enhanced leakage, ferroelectric and magnetic properties are observed in Zr doped ceramics. The analysis of microstructure and properties show that the cation vacancy defect plays an important role in modulating the electrical and magnetic properties of BiFeO3.

A phase field crystal model simulation of morphology evolution and misfit dislocation generation in nanoheteroepitaxy

A phase field crystal (PFC) model is employed to study morphology evolution of nanoheteroepitaxy and misfit dislocation generation when applied with enhanced supercooling, lattice mismatch and substrate vicinal angle conditions. Misfit strain that rises due to lattice mismatch causes rough surfaces or misfit dislocations, deteriorates film properties, hence, efforts taken to reveal their microscopic mechanism are significant for film quality improvement. Uniform islands, instead of misfit dislocations, are developed in subcritical thickness film, serving as a way of strain relief by surface mechanism. Misfit dislocations generate when strain relief by surface mechanism is deficient in higher supercooling, multilayers of misfit dislocations dominate, but the number of layers reduces gradually when the supercooling is further enhanced. Rough surfaces like islands or cuspate pits are developed which is ascribed to lattice mismatch, multilayers of misfit dislocations generate to further enhance lattice mismat...

Effect of CaCu3Ti4O12 modification on the structural, electrical and magnetic properties of BiFeO3 ceramics

ABSTRACT The polycrystalline samples of (1−x)BiFeO3-xCaCu3Ti4O12 (x = 0, 0.05, 0.10 and 0.15) were synthesized by solid state reaction method followed by rapid liquid phase sintering. The effects of CaCu3Ti4O12 (CCTO) doping on the microstructure, dielectric and magnetic properties of BiFeO3 (BFO) ceramics were studied by changing CCTO concentrations. The XRD analysis reveals that that a little amount of CCTO as a solid solution in the BFO phase leads to expanding the lattice, no phase transition has occurred in the doping content range, but CCTO doping induces the crystal structure distortion. Impedance spectroscopy results show that the doped BFO ceramics have a broadband stability of the dielectric constant and dielectric loss on frequency, the CCTO doping can improve the dielectric constant and reduce dielectric loss of BFO. It is also observed that an increase in CCTO doping concentration in BFO can improve the magnetic properties of BFO.

Influence of Gd/Eu Substitution on the Local Electron Density and the Superconductivity of Y1−xRxBa2Cu3O7−δ (R=Gd, Eu) Systems

The influences of Gd or Eu substitution on the local electronic density, lattice structure and the superconductivity are systemically studied by means of positron annihilation technique (PAT) and x‐ray diffraction (XRD) for Y1−xRxBa2Cu3O7−δ (R=Gd, Eu; x=0.0, 0.2, 0.5, 1.0) systems. XRD results show that all of the Y1−xRxBa2Cu3O7−δ samples have an orthorhombic phase as same as YBa2Cu3O7−δ (YBCO) sample. The positron annihilation experiments show that the mean positron lifetime increases with increasing Eu or Gd substitution content. The local electron density ne is evaluated as a function of the substitution content. The results indicate that the substitution ions in Y sites not only influence the local electron density around Y ion, but also change the distribution of electron density around Cu‐O chains. These results prove that the changes of ne, the degree of orthorhombic distortion and the coupling between the Cu‐O chains and the CuO2 planes all have an effect on the superconductivity of Y1−xRxBa2Cu3O7...

Spin Correlated Scattering and Metal‐Semiconductor Transition in CMR Manganites

The CMR manganites are one of the most interesting and widely noticed objects in the physical and materials sciences based on their possible application on CMR properties and sensor technology at low‐temperature, strong correlation characteristic, ordering of spin/charge/orbital including lattice degree of freedom. In this paper, a Kondo‐like transport was observed in the term of a metal‐semiconductor transition at low temperature in this ferromagnetic metallic phase system. The experiments show that this kind of Kondo‐like behavior can be tuned with an applied magnetic field and depends strongly on the content of a spin glass phase, which can be explained by the scattering of spin disorder or/and antiferromagnetic cluster in a micro‐scale. In undoped sample, the M‐SC transition would mean the existence of intrinsic spin disorder with magnetic inhomogeneities and gives a direct evidence of Kondo effect in ferromagnetic metallic manganites. It was also proved that Kondo‐like abnormity appears not only in t...