Fabrication and structure-property relationship of BaTiO3-Ni0.6Zn0.4Fe2O4 multiferroic composite

Abstract

In multiferroic materials, the grain size is an important factor for both ferroelectric and ferromagnetic phases. This research thus aimed at fabricating xBaTiO3 (BTO)/(1- x)Ni0.6Zn0.4Fe2O4 (NZFO) (where, x=0.7, 0.8 and 0.9) composites and obtain optimal microstructure with superior electrostrictive and magnetostrictive (multiferroic) properties. In this context, the xBaTiO3/(1-x)Ni0.6Zn0.4Fe2O4 composites (x= 0.7, 0.8, 0.9) were fabricated employing a conventional solid-state route. At first, NiFe2O4 (NFO) and ZnFe2O4 (ZFO) nanopowder were taken in stoichiometric ratio and pre-sintered at 9000C at 12 hr to form NZFO. Then, BTO nano-powder and NZFO pre-sintered powder were properly mixed in required proportion in planetary ball-mill and pressed into pallets. The fabricated pallets were sintered within a temperature range of 1250-13000C for different holding time. To identify phase and structure of the samples, X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were carried out. XRD confirmed the presence of tetragonal perovskite BTO and cubic spinel NZFO phases. However, microstructural observation using FESEM revealed that the grain size remained within the range of 100 nm in all samples. To increase the grain size, a different approach, termed as Batch-02in this thesis, was carried out, where BTO powder was pre-sintered at 9000C for 12 hr and NZFO powder was pre-sintered at 8500C for 12 hr. Both powders are mixed and then sintered at a temperature of 12500C for 4 hr resulted in significant increase in average grain size to 650 nm. The composite shows superior dielectric and ferromagnetic properties for the samples prepared in Batch-02. The best values of room temperature dielectric constant (~1834 at 1 kHz frequency) was attained by 0.9BTO/0.1NZFO ceramic sample prepared in Batch-02. The lack of oxygen vacancies in this sample provided high resistivity and thereby resulted in high dielectric constant. With increasing NZFO content dielectric constant was found to be decreased at the studied frequency range of 100 Hz to 2 MHz at room temperature. At higher temperatures, a considerable increase in the dielectric constant of all BTO/NZFO ceramic samples occurred due to space charge polarization. However, in 0.9BTO/0.1NZFO and 0.8BTO/0.2NZFO composites, the stability of dielectric constant with temperature was considerably improved due to lack ofoxygen vacancy in these samples. Curie temperature has shown to increase significantly with increasing composition of NZFO and broadened. A well defied ferroelecric loop (P-E loop) was found for all the composites. A positive curvature positive of P-E loop of 0.9BTO/0.1NZFO composite indicates a minimum leakage current contribution of the sample. With the increase of NZFO content in the composite, the curve drop sharply at the end of saturation polarization, indicating increased leakage current contribution in the composite. The I-V characteristics of the BTO/NZFO ceramics samples also showed the increase of leakage current densit with the increase of NZFO content within the composite which conform with the expected leakage of the composite found from P-E loop. The minimum leakage current density was found for 0.9BTO/0.1NZFO composite within the range of 10-7A/cm2 under 6kV/cm applied electric field. The ferromagnetic hysteresis loop (M-H loop) of the composites exhibit high values of saturation magnetization with low coercivity which is very typical for soft magnetic material. The maximum remnant magnetization of 5.87 emu/g was observed for 0.7BTO/0.1NZFO ceramic sample. However, a very interesting goose-neck type meta-magnetic hysteresis loop was obtained for 0.9BTO/0.1NZFO ceramic samples which can be attributed to the pinning effect creating by a large number small BTO grains surrounding the NZFO grains at BTO-NZFO interfaces to resist the rotation of the magnetic domain within NZFO phase.