Structural, Magnetic, and Dielectric Properties of Laser-Ablated CoFe 2 O 4 /BaTiO 3 Bilayers Deposited over Highly Doped Si(100)

Laser ablation was used to successfully fabricate multiferroic bilayer thin films, composed of BaTiO (BTO) and CoFe O (CFO), on highly doped (100) Si substrates. This study investigates the influence of BaTiO layer thickness (50-220 nm) on the films' structural, magnetic, and dielectric properties. The dense, polycrystalline films exhibited a tetragonal BaTiO phase and a cubic spinel CoFe O layer. Structural analysis revealed compression of the CoFe O unit cell along the growth direction, while the BaTiO layer showed a tetragonal distortion, more pronounced in thinner BTO layers. These strain effects, attributed to the mechanical interaction between both layers, induced strain-dependent wasp-waisted behavior in the films' magnetic hysteresis cycles. The strain effects gradually relaxed with increasing BaTiO thickness. Raman spectroscopy and second harmonic generation studies confirmed BTO's non-centrosymmetric ferroelectric structure at room temperature. The displayed dielectric permittivity dispersion was modeled using the Havriliak-Negami function combined with a conductivity term. This analysis yielded relaxation times, DC conductivities, and activation energies. The observed BTO relaxation time behavior, indicative of small-polaron transport, changed significantly at the BTO ferroelectric Curie temperature (Tc), presenting activation energies E in the 0.1-0.3 eV range for T < Tc and E > 0.3 eV for T > Tc. The BTO thickness-dependent Tc behavior exhibited critical exponents ν ~ 0.82 consistent with the 3D random Ising universality class, suggesting local disorder and inhomogeneities in the films. This was attributed to the composite structure of BTO grains, comprising an inner bulk-like structure, a gradient strained layer, and a disordered surface layer. DC conductivity analysis indicated that CoFe O conduction primarily occurred through hopping in octahedral sites. These findings provide crucial insights into the dynamic dielectric behavior of multiferroic bilayer thin films at the nanoscale, enhancing their potential for application in emerging Si electronics-compatible magneto-electric technologies.