Evidence of superparamagnetism and improved electrical properties in Ba and Ta co-doped BiFeO3 ceramics

Abstract

Ba and Ta co-doped bismuth ferrites with the composition Bi0.8Ba0.2Fe1-xTaxO3 (where x = 0, 0.05, 0.10 and 0.15) were synthesized employing a solid state reaction method. Rietveld refinement of XRD patterns revealed that Ba doping caused structural transformation in perovskite BFO from rhombohedral (R3c) to tetragonal (P4mm) with up to 10% Ta doping. However, with 15% Ta substitution (BBFTO-15) the ceramics exhibited a mixture of tetragonal and rhombohedral structures. Curve fitting of room temperature magnetization vs. magnetic field data showed that the shape of the hysteresis loops for these ceramics were the result of coexistence of single domain and superparamagnetic contributions. For 10% Ta substitution (BBFTO-10), the contribution from superparamagnetic particles was dominant. Polarization with respect to electric field measurements did not yield typical hysteresis loops; instead evidence of high leakage was present in BBFTO-0 and BBFTO-5 while that of BBFTO-10 and BBFTO-15 exhibited a lossy dielectric nature. The I-V characteristics confirmed that Ta doping reduced the leakage current by greater than two orders of magnitude compared to BBFTO-0. It is believed that the reduction in oxygen vacancy in the doped samples was key in reducing the leakage current. As a matter of fact, XPS data revealed a reduction in oxygen vacancy concentration and the percentage of Fe3+was also found to increase with increasing Ta doping. Dielectric constant and dielectric loss were also found to decrease with the increase of Ta doping. Impedance spectroscopy confirmed the contribution from multiple impedance components including evidence of a dipolar relaxation that was found in BBFTO-5 and BBFTO-10. Defect associates were proposed to be responsible for such relaxations.