Structural and multiferroic properties of BiFeO3 thin films at room temperature

${\mathrm{BiFeO}}_3$ thin films have been prepared on $\mathrm{Pt}∕{\mathrm{TiO}}_2∕{\mathrm{SiO}}_2∕\mathrm{Si}$ substrates under various oxygen pressures of $0.15–0.005\mathrm{Torr}$ at a temperature of $450°\mathrm{C}$ by pulsed-laser deposition. The effects of deposition pressure on their crystal structure and multiferroic properties have been investigated at room temperature. X-ray diffraction analysis ($\theta \text{-}2\theta$ scans and 2-dimensional scans) shows that the ${\mathrm{BiFeO}}_3$ thin films consist of perovskite single phase with tetragonal crystal structure and space group $P4mm$ . The c-axis lattice constant decreases $(4.062–4.006\mathrm{\AA })$ and $c∕a$ ratio of the films decreases from 1.032 to 1.014 with a decrease in the oxygen pressure. The surface roughness and grain size of the films depend dramatically on oxygen pressures. The dielectric constant of the films decreases with decreasing oxygen pressure. The film deposited at $0.05\mathrm{Torr}$ shows a stable current density and well-saturated hysteresis loop with twice the remanent polarization $\left(2P_r\right)$ of $136\mu \mathrm{C}∕{\mathrm{cm}}^2$ and coercive field $\left(2E_c\right)$ of $109\mathrm{kV}∕\mathrm{cm}$ . The ${\mathrm{BiFeO}}_3$ thin films also show the saturated weak ferromagnetic hysteresis loops with a small remanent magnetization.