Enhanced multiferroic properties of Co_0.5Ni_0.5Fe₂O₄/Ba_0.85Sr_0.15TiO₃ composites based on particle size effect

Co_0.5Ni_0.5Fe₂O₄(CNFO) were prepared by chemical coprecipitation method and the particles were tuned by varying annealing temperature (400, 500, 600, 700 and 800 °C), and on this basis, CNFO/Ba_0.85Sr_0.15TiO₃ (BST) composites were prepared by sol–gel method. Effects of CNFO particle size on the microstructure, dielectric and multiferroic properties were systematically investigated and discussed. The results confirm pure CNFO and BST phases, without any obvious impurity phases. The particle size of CNFO is in the range of 20–50 nm, and all the composites show relatively uniform grain size except some pores presented on the surface. The dielectric constant and loss decrease with the particle size as a whole, while the permittivity shows strong dependence of the measuring frequency due to the relaxation polarization mechanism, of which the relaxation peak decreases and shifts to high temperature range with the frequency. The polarization increases with the testing voltage but decreases with the test frequency increasing, the remnant polarization decreases with the increase of particles size, the highest value is 2.2 μC/cm², obtained at 100 Hz for the sample with the smallest particle size. The maximal remnant magnetization is 11.2 emu/g, which is also obtained when the particle size is the smallest due to its strongest interface interaction. The ME coefficient decreases with increasing the particles size and the maximum ME coefficient is about 13.2 mV/(cm Oe). These results provide valuable information for improving the magnetoelectric properties of composites by tuning the particles size.