J. W. Hubbard

Bio: J. W. Hubbard is an academic researcher from University of Puerto Rico at Mayagüez. The author has contributed to research in topics: Dielectric & Néel temperature. The author has an hindex of 1, co-authored 1 publications receiving 65 citations.

Dielectric anomalies due to grain boundary conduction in chemically substituted BiFeO3

Abstract: We describe systematic studies on Nd and Mn co-doped BiFeO3, i.e., (Bi0.95Nd0.05)(Fe0.97Mn0.03)O3 (BNFM) polycrystalline electroceramics. Raman spectra and X-ray diffraction patterns revealed the formation of rhombohedral crystal structure at room temperature, and ruled out structural changes in BiFeO3 (BFO) after low percentage chemical substitution. Strong dielectric dispersion and a sharp anomaly around 620 K observed near the Neel temperature ( TN ∼ 643 K of BFO) support strong magneto-dielectric coupling, verified by the exothermic peak in differential thermal data. Impedance spectroscopy disclosed the appearance of grain boundary contributions in the dielectric data in the region, and their disappearance just near the Neel temperature suggests magnetically active grain boundaries. The resistive grain boundary components of the BNFM are mainly responsible for magneto-dielectric coupling. Capacitive grain boundaries are not observed in the modulus spectra and the dielectric behavior deviates from the ...

Exploring the Magnetoelectric Coupling at the Composite Interfaces of FE/FM/FE Heterostructures.

Abstract: Multiferroic materials have attracted considerable attention as possible candidates for a wide variety of future microelectronic and memory devices, although robust magnetoelectric (ME) coupling between electric and magnetic orders at room temperature still remains difficult to achieve. In order to obtain robust ME coupling at room temperature, we studied the Pb(Fe0.5Nb0.5)O3/Ni0.65Zn0.35Fe2O4/Pb(Fe0.5Nb0.5)O3 (PFN/NZFO/PFN) trilayer structure as a representative FE/FM/FE system. We report the ferroelectric, magnetic and ME properties of PFN/NZFO/PFN trilayer nanoscale heterostructure having dimensions 70/20/70 nm, at room temperature. The presence of only (00l) reflection of PFN and NZFO in the X-ray diffraction (XRD) patterns and electron diffraction patterns in Transmission Electron Microscopy (TEM) confirm the epitaxial growth of multilayer heterostructure. The distribution of the ferroelectric loop area in a wide area has been studied, suggesting that spatial variability of ferroelectric switching behavior is low, and film growth is of high quality. The ferroelectric and magnetic phase transitions of these heterostructures have been found at ~575 K and ~650 K, respectively which are well above room temperature. These nanostructures exhibit low loss tangent, large saturation polarization (Ps ~ 38 µC/cm2) and magnetization (Ms ~ 48 emu/cm3) with strong ME coupling at room temperature revealing them as potential candidates for nanoscale multifunctional and spintronics device applications.

Optimisation of functional properties in lead-free BiFeO3–BaTiO3 ceramics through La3+ substitution strategy

Abstract: The paper is concerned with the structure and functional properties of bismuth ferrite–barium titanate, 0.75BiFeO3–0.25BaTiO3, solid solutions. Such materials are attracting attention due to their potential applications in high temperature piezoelectric transducers for use in demanding environments in process monitoring, for example. The article focuses on the mechanism of incorporation of the minor dopant lanthanum oxide, either in the form of isovalent or donor-type substitution. It is shown that the development of chemical heterogeneity in the form of core–shell grain microstructures, linked to donor-type substitution, plays a key role in controlling the functional behaviour. The use of an air-quenching procedure results in dramatic improvements in the ferroelectric properties, accompanied by a transformation of the shell regions from a pseudo-cubic, Pmm, to rhombohedral, R3c, structure. These observations are interpreted in terms of a novel mechanism involving nanoscale phase separation in the ‘shell’ regions during slow cooling, which impedes the development of ferroelectric ordering and leads to the formation of a nano-polar relaxor ferroelectric state. The work highlights the importance of immiscibility in bismuth-based ferroelectric perovskite solid solutions and illustrates how their ferroelectric, piezoelectric, dielectric energy storage, ferromagnetic and magneto-electric properties can be tuned by consideration of the substitution mechanism and control of thermal processing parameters.

Chemical heterogeneity and approaches to its control in BiFeO3–BaTiO3 lead-free ferroelectrics

Abstract: 1 mol% MnO2 was used to improve electrical resistivity of lead-free 0.75BiFeO3–0.25BaTiO3 (75BFBT) ferroelectric ceramics; the materials were perovskite structured with major rhombohedral (R3c) phase. The method of incorporation of MnO2 was found to exert a significant influence on the structure, microstructure and electrical properties. Chemical heterogeneity in the form of core–shell grain microstructures was observed when MnO2 was added into the undoped calcined powder, in contrast to the relatively homogeneous materials that resulted from adding MnO2 into the precursor oxide mixture prior to calcination. Compositionally graded regions were detected across the grains consisting of a BF-rich core and BF-depleted shell. The occurrence of core–shell type microstructures led to various characteristic features including a high cubic phase fraction, contrast between ordered ferroelectric domain configurations in the rhombohedral core and the relatively featureless pseudo-cubic shell, constrained ferroelectric domain switching, and two distinct anomalies in dielectric permittivity at temperatures of 485 and 635 °C. The latter features are attributed to separate phase transitions in the relaxor ferroelectric shell and normal ferroelectric core regions respectively. The application of a thermal quenching procedure caused the formation of ferroelectric domain structures throughout the microstructure and resulted in dramatically enhanced ferroelectric switching behaviour. For example, the remnant polarisation of the as-sintered 75BFBT ceramic increased from 0.06 to 0.31 C m−2 after air-quenching. These effects are tentatively attributed to nanoscale phase segregation in the shell region of the as-sintered ceramics, resulting from thermodynamic immiscibility between the BF and BT solid solutions.

Studies of Phase Transitions and Magnetoelectric Coupling in PFN-CZFO Multiferroic Composites

Abstract: We report studies of the ferroelectric and magnetic phase transitions of (1 – x)Pb(Fe0.5Nb0.5)O3 – xCo0.65Zn0.35Fe2O4 (x = 0.2) composite with emphasis upon the nature of magnetoelectric coupling at room temperature. The presence of all cationic elements with their required stoichiometry have been confirmed by SEM and XPS studies. The composite shows well-saturated ferroelectric and ferromagnetic (multiferroic) behavior at room temperature. A ferroelectric-paraelectric phase transition has been confirmed from the temperature dependent dielectric spectra along with DSC and Raman spectroscopic studies. Antiferromagnetic, ferromagnetic, and relaxor paramagnetic states have been observed in this composite. This composite shows strong bulk biquadratic magnetoelectric coupling at room temperature, which can be useful for potential multifunctional device applications.