DC and AC Conductivity of CoFe₂O₄/BaTiO₃ Bilayers Deposited Over Nb-Doped SrTiO₃(100) Substrates

Multiferroic BaTiO₃ (BTO, piezoelectric)/CoFe₂O₄ (CFO, magnetostrictive) bilayer thin films were prepared by laser ablation on conductive Nb-doped SrTiO₃ (100) substrates to investigate the influence of BTO layer thickness on their structural, microstructural, dielectric, and electrical (DC and AC) properties. X-ray diffraction confirmed the coexistence of the cubic spinel CoFe₂O₄ phase and the tetragonal ferroelectric BaTiO₃ phase. The films exhibit preferred orientation, with CFO showing the [400] direction along the growth axis and BTO displaying (100)/(001) planes stacked parallel to it. The CFO unit cell is compressed along the growth direction, while BTO presents the ferroelectric distortion with a tetragonality ratio (c/a) slightly below, but close to, the bulk value. Second harmonic generation studies further verified the non-centrosymmetric ferroelectric nature of BTO at room temperature. The temperature-dependent dielectric permittivity was modeled using the Havriliak–Negami function with an additional conductivity term to extract relaxation dynamics, DC conductivity, Curie temperature (Tc), and activation energies. The Curie temperature increases with BTO thickness, approaching the bulk value for thicker layers. DC conductivity activation energies exhibit a change at Tc, from below 0.5 eV for T < Tc to above 0.5 eV for T > Tc, consistent with small-polaron tunneling. The AC conductivity follows a Jonscher-type frequency dependence with two power-law contributions reflecting the behavior of both layers. Temperature-dependent analysis of the power-law exponents reveals that small-polaron tunneling dominates conduction in BTO, while ion hopping between octahedral sites governs conduction in CFO. Underoxidation leads to a more complex transport regime in BTO, showing a transition from quantum-mechanical tunneling below Tc to correlated barrier hopping above it. By revealing how transport processes operate within multiferroic oriented bilayer systems, these findings advance our understanding of material interactions and pave the way for the design of innovative multifunctional platforms optimized for spintronic technologies.