Sr_0.5Ba_0.5Nb₂O₆/Ni_0.8Zn_0.2Fe₂O₄-based Magnetoelectric Composite Ceramics

The magnetoelectric (ME) effect is defined as the dielectric polarization of a material in an applied magnetic field, or an induced magnetization in an external electric field. This effect would make the conversion between electric energy and magnetic energy possible, which provides opportunities for potential applications as ME memories, waveguides, transducers, actuators, and sensors. In the present work, the magnetoelectric composites, with a typical tungsten-bronze relaxor Sr_0.5Ba_0.5Nb₂O₆, as ferroelectric phase and a cubic spinel structural ferrite, Ni_0.8Zn_0.2Fe₂O₄ as ferromagnetic phase, are prepared and characterized to explore new magnetoelectric materials.Novel magnetoelectric composites were prepared by incorporating the dispersed Ni_0.8Zn_0.2Fe₂O₄ ferromagnetic particles into Sr_0.5Ba_0.5Nb₂O₆ relaxor ferroelectric matrix. Dense composite ceramics were obtained with the co-presence of Sr_0.5Ba_0.5Nb₂O₆ and Ni_0.8Zn_0.2Fe₂O₄, and they could be electrically and magnetically poled to exhibit a significant magnetoelectric effect. A maximum magnetoelectric voltage coefficient of 26.6mV/cm/Oe was obtained from the composite with 70mol% Sr_0.5Ba_0.5Nb₂O₆, which was much greater than that of single phase magnetoelectric compounds and solid solutions. The ME coefficient of the present composites varied significantly with sintering temperature, and therefore the enhanced dE/dH could be expected through processing and microstructure controlling.Moreover, an interesting dielectric relaxation and associated high permittivity were observed in the composites, which was attributed to the space charge pile-up at the interfaces when an electric current passed through interfaces between two different dielectric media. The dielectric relaxation and associated high permittivity sustains the composite to be a promising candidate for giant dielectric constant materials. An ideal equivalent circuit was used to explain the electrical responses in impedance formalism. A Debye-like relaxation in the permittivity formalism was alsofound. Moreover, real permittivity (s') of the sample with 30mol% Nio.8Z showed the temperature independence at 100 kHz. Dielectric relaxation and high-s' of the composites were explained in terms of the Maxwell-Wagner (MW) polarization model.