High-Performance, Lead-Free Magnetoelectric Composites Based on Nickel–Cobalt Ferrite and Calcium/Zirconium-Substituted Barium Titanate

Herein we report on the synthesis, chemistry, and structure–property correlation of magnetoelectric (ME) composites, where the rare earth (RE) ion-substituted nickel–cobalt (Ni–Co) mixed ferrite serves as the magnetic phase and the Pb-free, Ca/Zr-doped BaTiO3 serves as the ferroelectric and piezoelectric phase. The magnetostrictive–piezoelectric ME composites with variable composition, namely, 0.9(BaZr0.04Ti0.96O3)–0.1(Co0.9Ni0.1Fe2O4), 0.9(BaZr0.04Ti0.96O3)–0.1(Co0.9Ni0.1Fe1.95Dy0.05O4), 0.9(Ba0.92Ca0.08Zr0.04Ti0.96O3)–0.1(Co0.9Ni0.1Fe2O4), and 0.9(Ba0.92Ca0.08Zr0.04Ti0.96O3)–0.1(Co0.9Ni0.1Fe1.95Dy0.05O4), were prepared by the conventional standard solid-state chemical reaction method. These complex materials were investigated to understand their structure, morphology, and ferroelectric, magnetic, dielectric, and magnetoelectric properties and performance. X-ray diffraction (XRD) and Rietveld refinement analyses confirmed the purity of the ferroelectric and magnetic phases, while the polarization (P) versus electric field (E) measurements revealed the ferroelectric-like nature of all of the ME composites. Equally, the inverse piezoelectric effect was confirmed by means of bipolar strain versus electric field measurements, i.e., S–E loop measurements. Maximum strain (% strain) was observed for the 0.9(BaZr0.04Ti0.96O3)–0.1(Co0.9Ni0.1Fe2O4) ME composite. Magnetization versus magnetic field (M–H) hysteresis measurements validated the magnetic nature of all of the ME composites. The magnetic parameters, viz., saturation magnetization (Ms), remnant magnetization (Mr), and coercive field (Hc), decrease with increasing temperature. The magnetization versus temperature (M–T) measurements govern the Curie temperature of the magnetic phase present in the ME composite, where the composite material loses its magnetic nature and becomes paramagnetic. The variation of the dielectric constant (ε) with frequency indicates typical dielectric dispersion behavior, while the ε–T curve displays the transition temperature of the ferroelectric phase present in the ME composite. The grain-interior conduction mechanism is evident in T-dependent impedance measurements, where the presence of a single semicircle is seen in Cole–Cole plots. Furthermore, the structure–property correlation and ME voltage coefficient (αME) measurements demonstrate that the ME composite with composition 0.9(BaZr0.04Ti0.96O3)–0.1(Co0.9Ni0.1Fe2O4) possesses the maximum αME of 2027 μV cm–1 Oe–1, which makes it a candidate material suitable for the design of magnetoelectric sensing devices.