Influence of the Preparation Conditions and Percolation Threshold on the Properties of Lead Zirconate Titanate/Cobalt Nickel Ferrite Magnetoelectric Composites

We have prepared magnetoelectric (ME) composite ceramics, free of foreign phases, in the lead zirconate titanate–cobalt nickel ferrite two-phase system: xPZT-36 + (100–x)Ni_0.9Co_0.1Fe₂O₄. The sol–gel derived ferrite powder used in our preparations seems to be doped with titanium cations from the PZT-36. The ceramics have a percolation threshold at x = 50–70 wt %, which is due to the increased electrical conductivity of Ni_0.9Co_0.1Fe₂O₄. As a consequence, the piezoelectric parameters of the ME ceramics drop sharply at x < 50–70 wt %: the piezoelectric moduli |d_ij| and piezoelectric voltage coefficients |g_ij| decrease by a factor of 3–5 in this composite range. The piezoelectric parameters |d_ij| and |g_ij| of the composites produced using the fine ferrite powder exceed those of the materials prepared using macrocrystalline Ni_0.9Co_0.1Fe₂O₄ powder by more than a factor of 2. The piezoelectric voltage coefficient g33 correlates with the ME coefficient ΔE/ΔH. The highest ME conversion efficiency (up to 45 mV/(cm Oe)) is offered by the 80 wt % PZT-36 + 20 wt % Ni_0.9Co_0.1Fe₂O₄ composites, whose composition lies in a subpercolation region. Even though the composites produced using the fine ferrite powder possess improved piezoelectric properties, they have smaller ΔE/ΔH coefficients (no greater than 25 mV/(cm Oe)), which can be tentatively attributed to the degradation of the properties of the ferrite as a consequence of doping with Ti⁴⁺ cations during the sintering of the composite ceramics.