General aspects of the physical behavior of polycrystalline BiFeO${}_3$/VO${}_2$ bilayers grown on sapphire substrates

Heterostructures composed of bismuth ferrite, BiFeO${}_3$, and vanadium dioxide, VO${}_2$, films were successfully grown by means of the radio-frequency and DC sputtering on sapphire substrates. The X-ray diffraction (XRD) patterns showed that both films were polycrystalline in nature. The XRD data revealed the formation of the monoclinic VO${}_2$(M) phase. The bottom VO${}_2$ exhibited a sharp metal-insulator transition (MIT) at ~ 327 K with a variation in the value of the resistance of two orders of magnitude. Measurements of the electric polarization (P) versus the electric field (E) of the top BiFeO${}_3$ showed closed loops, although visually distinct from those of a true ferroelectric. In spite of the well-defined MIT of VO${}_2$, the effect of the structural change associated with the transition on the ferroelectric properties of BiFeO${}_3$ was masked by the large leakage currents. The probably perturbation of the ferroelectric order in the BiFeO${}_3$, associated with the structural changes through the MIT of VO${}_2$ was investigated by measuring the P-E dependence at temperatures below and above the MIT of VO${}_2$. Although round-shaped hysteresis were obtained, a small but sizeable difference in the values of the apparent remnant P at the temperatures of interest was observed. By the same token, the transition of VO${}_2$ seemed to influence the possible magneto-electric coupling in the BiFeO${}_3$ layer.