High-Frequency Electric Field-Induced Polarization Response in Barium Titanate Thin Films

We used molecular dynamics simulations to investigate the frequency-dependent polarization-electric field (P-E) response of BaTiO₃ nanofilms for in-plane and out-of-plane E field/film orientation. Our study reveals that the polarization direction (in-plane or out-of-plane) of thin films profoundly impacts polarization-switching behavior across a broad frequency range. Out-of-plane polarized films exhibit rather distinct behavior at all frequencies compared to in-plane polarized films and bulk BaTiO₃. In particular, at the lowest studied frequencies (∼10 GHz), these films behave like paraelectric materials, showing negligible P-E hysteresis. This behavior is due to depolarizing fields and surface-induced barrierless polarization switching. In contrast, in-plane polarized thin films, as thin as ∼10 nm, exhibit frequency-dependent ferroelectric characteristics and are similar to those characterizing bulk BaTiO₃. At low frequencies (10-200 GHz), they show well-defined square-type P-E ferroelectric loops and transition to elliptical shapes with reduced remnant polarization at higher frequencies (500-1000 GHz). Our findings demonstrate that the polarization direction in thin films significantly influences frequency-dependent ferroelectric switching dynamics, offering a pathway for the design of ultrathin ferroelectric materials operating across a wide frequency spectrum.