Relation between the microstructure and the electromagnetic properties of BaTiO3/Ni0.5Zn0.5Fe2O4 ceramic composite

The microstructure–property relation in ferroelectric/ferromagnetic composite is investigated in detail, exemplified by typical sol–gel-derived 0.3BTO/0.7NZFO ceramic composite. The effect of microstructural factors including intergrain connectivity, grain size and interfaces on the dielectric and magnetic properties of the composite prepared by conventional ceramic method and three-step sintering method is discussed both experimentally and theoretically. It reveals that the dielectric behavior of the composite is controlled by a hybrid dielectric process that combines the contribution of Debye-like dipoles and Maxwell–Wagner (M–W or interfacial) polarization. Enhanced dielectric, magnetic and conductive behaviors appear in the composite with better intergrain connectivity and larger grain size derived by sol–gel route and three-step sintering method. The effective permittivity contributed by Debye-like dipoles exhibits a value of ~130,000 in three-step sintered composite, which is almost the same as that in conventionally sintered one, but that contributed by M–W response is much smaller in the former. Compared with conventionally prepared samples, the relaxation time (τ) is 3.476 × 10−6 s, about one order of magnitude smaller, and the dc electrical conductivity is 3.890 × 10−3 S/m, one order of magnitude higher in three-step sintered composite. The minimum dielectric loss reveals almost the same (~0.2) for all samples, but shows distinguishable difference in low-frequency region. Meanwhile, an initial permeability of 84, twice as large as that of conventionally prepared composite and 56 % the value of single-phased NZFO ferrite (~150), and a saturation magnetization of 63.5 emu/g, 32 % higher than that of conventional one and approximately 84 % the value of single-phased NZFO ferrite (~76 emu/g), appear simultaneously in three-step sintered composite with larger grain size and better intergrain connectivity. It is clear that the discovery is helpful for establishing a more explicit view on the physics of multi-functional composite materials, while the composite with optimized microstructure is beneficial to be used as a high-performance material.