Unlocking Strong Zero-Bias Magnetoelectric Effects in Bilayers of PZT and Self-Biased Hexagonal Ferrite–Spinel Ferrite Composites

Ferromagnetic-ferroelectric composites are of interest for studies of magneto-electric (ME) coupling between the two phases, which is mediated by mechanical strain arising from magnetostriction and the piezoelectric effect. A strong ME response in such composites typically requires an external DC bias magnetic field to compensate for weak zero-bias piezomagnetic deformation in the ferromagnetic phase. This reliance on an external field remains a key barrier to device miniaturization. Here we report strong, self-biased ME coupling achieved in a bilayer structure consisting of lead zirconate titanate (PZT) and a dual-phase ferromagnetic composite. The composite pairs hard barium hexaferrite BaFe12O19 (BaM), with its high magneto-crystalline anisotropy field, with soft nickel-zinc spinel ferrite Ni0.8 Zn0.2 O4 (NZFO), with its high magnetostriction. Samples consisting of (100-x) wt.% BaM- x wt.% NZFO (BNZx) were prepared via conventional solid-state sintering, and x-ray diffraction confirmed phase purity. Magnetic characterization revealed an in-plane anisotropy field (HA) ranging from 4.8 kOe to 8.3 kOe, which decreased systematically with increasing spinel content, alongside a high saturation magnetostriction of -25 ppm. The ME coupling strength of the BNZx-PZT bilayers was evaluated via the ME voltage coefficient (MEVC) across both low-frequency and electromechanical resonance regimes. The MEVC increased markedly with higher x-values, peaking at x=90. Under zero external DC bias, the bilayer showed a low-frequency (100 Hz) MEVC of 114 mV/cm Oe and a remarkably high MEVC of 3.6 V/cm Oe at resonance (10.6 kHz). The corresponding MEVC values under an external bias were 161 mV/cm Oe and 6 V/cm Oe. This pronounced self-biased behavior, driven by the internal field of the hard BaM phase, eliminates the need for external biasing elements and provides a practical framework for compact magnetic sensors, imaging arrays, and energy harvesters.