O. Raymond

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part I. Structural and thermo-electrical characterization

With the purpose of fabricating ceramics where ferroelectric and magnetic order coexist, ceramics of Pb(Fe1∕2Nb1∕2)O3 have been prepared using the traditional ceramic method following three different routes. The first is a direct via starting from oxide reagents and the other two use different kinds of FeNbO4 precursors with either monoclinic or orthorhombic structures. Crystallographic and surface morphological studies were carried out by the powder x-ray diffraction and scanning microscopy techniques. The presence of Fe2+, detrimental to the ferroelectric and magnetic performance, was evaluated by x-ray photoelectron spectroscopy. The samples showed no structural differences, uniformly distributed grains, a ferro-paraelectric transition temperature at 110°C and a normal diffuse phase transition (nonrelaxor behavior). Differences in the degree of diffuseness, densities and grain size were observed depending on the kind of precursor. Measurements of dc and ac electrical resistivity, dielectric constant an...

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part II. Impedance spectroscopy characterization

The dielectric behavior of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained using the traditional ceramic method employing three different precursors was investigated by impedance spectroscopy in the temperature range of 200–300°C. This study was carried out by means of the simultaneous analysis of the complex impedance Z, electric modulus M, and admittance Y functions from the measurements in the frequency range of 20Hz–1MHz. In correspondence to a previous structural, morphological, and temperature response study, appropriate microstructural and equivalent circuit models were established. Based on the brick layer model, three series of interconnected electrically distinct regions are considered: a conductive grain boundary layer, a capacitive grain boundary surface layer, and a resistive-ferroelectric bulk layer. Two conduction mechanisms were identified: a dielectric relaxation process due to localized conduction associated with the presence of oxygen vacancies and the nonlocalized conduction...

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. III. Dielectric relaxation near the transition temperature

Dielectric relaxation processes occurring near the ferroelectric-paraelectric phase transition of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors are discussed using microstructural and equivalent circuit modeling and the impedance spectroscopy technique. The frequency-temperature response was obtained from room temperature to 300°C and from 20Hzto1Mz. In correspondence with a previous structural, morphological, and temperature response study, appropriate microstructural, and equivalent circuit models were established. The frequency response study was carried out by means of the simultaneous analysis of the complex dielectric constant e and admittance Y functions and the dielectric loss, tanδ. A strong absorption near the transition temperature region at a frequency around 1MHz is discussed and is attributed with relaxation processes associated with domain reorientation, domain wall (DW) motion, and the dipolar behavior of ferroelectric materials. Such processes were foun...

Effects of two kinds of FeNbO4 precursors in the obtainment and dielectric properties of PFN ceramics

We report here a study of the obtainment, structural and dielectric characterization of the monoclinic and orthorhombic phases of FeNbO4 precursors and their effects in the processing and the dielectric properties of Pb(Fe1/2Nb1/2)O3 ceramics. Crystallographic and surface morphological studies were carried out by powder X-ray diffraction and scanning electron microscopy techniques. For structural determination, runs of Rietveld refinement were realized employing FULLPROF software. Measurements of electrical resistivity, dielectric constant and dielectric loss as functions of temperature were realized in PFN ceramics. An analysis and a comparative discussion of results are presented.

Highly textured Sr, Nb co-doped BiFeO3 thin films grown on SrRuO3/Si substrates by rf- sputtering

In this study, (011)-highly oriented Sr, Nb co-doped BiFeO3 (BFO) thin films were successfully grown on SrRuO3/Si substrates by rf-magnetron sputtering. The presence of parasite magnetic phases was ruled out based on the high resolution x-ray diffraction data. BFO films exhibited a columnar-like grain growth with rms surface roughness values of ≈5.3 nm and average grain sizes of ≈65–70 nm for samples with different thicknesses. Remanent polarization values (2Pr) of 54 μC cm−2 at room temperature were found for the BFO films with a ferroelectric behavior characteristic of an asymmetric device structure. Analysis of the leakage mechanisms for this structure in negative bias suggests Schottky injection and a dominant Poole-Frenkel trap-limited conduction at room temperature. Oxygen vacancies and Fe3+/Fe2+ trap centers are consistent with the surface chemical bonding states analysis from x-ray photoelectron spectroscopy data. The (011)-BFO/SrRuO3/Si film structure exhibits a strong magnetic interaction at the...

ac conductivity in Gd doped Pb(Zr0.53Ti0.47)O3 ceramics

This study is focused in the conduction processes taking place in 0.6 wt % Gd doped lead zirconate titanate samples PbZr0.53Ti0.47O3:Gd (PZT53/47:Gd) in the vicinity of the morphotropic phase boundary. Doped samples show very large dielectric permittivity with respect to that of undoped ones near the transition temperature. The frequency dependent ac conductivity of PZT53/47:Gd ceramics was studied in the 30–450 °C temperature range. X-ray diffraction analyses indicate the incorporation of Gd atoms to the structure. The changes in the dielectric properties as functions of temperature of the doped samples are taken as additional evidence of the incorporation of Gd into the crystal structure. Gd acts as donor center promoting extrinsic n-type conduction. The ac conductivity behavior obeys Jonscher universal relation in the 100 Hz–1 MHz frequency range for temperatures between 30 and 300 °C. The measured conductivity values for Gd doped PZT53/47 are higher than those of pure PZT53/47. According to the correl...

Growth and characterization of ZnS and ZnCdS nanoclusters in mordenite zeolite host

Abstract The growth of semiconductor nanoclusters of ZnS and the ternary ZnCdS system embedded in synthetic mordenites by ionic exchange using different mixtures of CdCl 2 and ZnCl 2 as starting sources, followed by treatment in hydrogen sulphide flow, is reported. Compositional, morphological and structural studies were realized by XRD, SEM, TEM and energy dispersive spectroscopy (EDS). XRD shows well-crystallized mordenite and the absence of secondary phases; EDS gives us the chemical composition of the matrix host and the relative weight percent of Cd, Zn and S. TEM and SEM show the morphology, packing and sizes of mordenite crystals (bundled needles of 3 μm long and 0.07 μm width). Measurements of the absorption spectra by diffuse reflectance spectroscopy in the UV-visible range allow us an analysis of the optical behavior as a function of the relative concentration of Zn and Cd. Samples show blue shifts in the absorption edges when the Cd concentration decreases and correspondingly, the Zn concentration increases. The optical band gap is determined and its behavior with concentration changes is discussed. Previously, ZnCdS clusters supported on mordenite were prepared in our laboratory by hydrothermal growth. A comparative discussion of the results obtained through both synthetic processes is presented.

Magnetoelectric coupling study in multiferroic Pb(Fe0.5Nb0.5)O3 ceramics through small and large electric signal standard measurements

Multifunctional multiferroic materials such as the single phase compound Pb(Fe0.5Nb0.5)O3 (PFN), where ferroelectric and antiferromagnetic order coexist, are very promising and have great interest from the academic and technological points of view. In this work, coupling of the ferroelectric and magnetic moments is reported. For this study, a combination of the small signal response using the impedance spectroscopy technique and the electromechanical resonance method with the large signal response through standard ferroelectric hysteresis measurement, has been used with and without an applied magnetic field. The measurements to determine the electrical properties of the ceramic were performed as functions of the bias and poling electric fields. A simultaneous analysis of the complex dielectric constant ɛ, impedance Z, electric modulus M, admittance Y, and the electromechanical parameters and coupling factors is presented. The results are correlated with a previous study of structural, morphological, s...

Evidence of magnetodielectric coupling in multiferroic Pb"Fe0.5Nb0.5…O3 ceramics from ferroelectric measurements and electron paramagnetic resonance

Magnetodielectric coupling effects of single phase multiferroic Pb(Fe0.5Nb0.5)O3 ceramics have been studied using standard ferroelectric measurements and electron paramagnetic resonance (EPR). Changes in the electrical polarization behavior were observed in the paramagnetic to weakly magnetized antiferromagnetic transition near 103 K associated with a rhombohedral to monoclinic symmetry reduction. The analysis of changes in the EPR spectral parameters confirms the transition from paramagnetic to weakly magnetized antiferromagnetic and reveals noticeable anomalies in the high temperature region near the ferroelectric-paraelectric transition (383 K), which are correlated with the tetragonal-to-cubic symmetry change and the characteristic diffuse phase transition of this material.

Ferroelectric hysteresis and aging effect analysis in multiferroic Pb(Fe0.5Nb0.5)O3 ceramics

Ferroelectric hysteresis and aging effects in multiferroic Pb(Fe0.5Nb0.5)O3 ceramics obtained through different precursors are discussed using the hysteresis loop modeling by equivalent circuit. Maximum polarization Pmax, remanent polarization Pr, and coercive field EC as functions of external electric fields are determined. A comparative analysis of Pmax, Pr, and EC parameters obtained from the full and remanent hysteresis loops is presented and correlated with a previous structural, morphological, and small signal dielectric frequency-temperature response study using the impedance spectroscopy technique. Three contributions to the full hysteresis were identified: first, a dipolar contribution; second, space charge movement; and third, orientation of the switchable ferroelectric domains. The smaller contribution of the switchable ferroelectric domains, in contrast to the high polarizability of the compound, is explained by assuming that the grains have a complex inner polydomain structure with a distribu...