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.