Abstract
Epitaxial multiferroic BaTiO3-BiFeO3 composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient αME and the oxygen partial pressure during growth. The ME coefficient αME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of αME of the composite films is opposite that of recently-reported BaTiO3-BiFeO3 superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO3-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO3-BiFeO3 composite system and, thus, paves the way to practical exploitation of magnetoelectric composites.
Highlights
Multiferroic composites consisting of two different chemical compounds offer unique flexibility in geometrical and structural design to achieve desired functional properties, in particular a high magnetoelectric coupling [1,2]
The substrate material of the films intended for X-ray diffraction (XRD) and ferroelectric and magnetoelectric measurements was SrTiO3 :Nb(001)
Two composite film samples grown on MgO(001) were investigated by scanning transmission electron microscopy (STEM) and selected area electron diffraction (SAED), Two composite film samples grown on MgO(001) were investigated by STEM and SAED, namely namely one sample grown at 0.01 mbar and the other at 0.25 mbar one sample grown at 0.01 mbar and the other at 0.25 mbar
Summary
Multiferroic composites consisting of two different chemical compounds offer unique flexibility in geometrical and structural design to achieve desired functional properties, in particular a high magnetoelectric coupling [1,2]. We reported the effect of rare-earth doping on the multiferroic properties of BiFeO3 thin films [21] In continuation of this latter work on BiFeO3 , we found that the ME coefficient of single-phase. While a BiFeO3 film showed an αME of about 2 V/(cm Oe), the corresponding (BaTiO3 -BiFeO3 ) ˆ 15 superlattice showed 9 V/(cm Oe) at 300 K, clearly demonstrating the interface effect on the magnetic moment direction and magnetoelectric coupling, as published in [23]. In our first chemically-homogeneous composite films published in [11], we combined the ferroelectric BaTiO3 and multiferroic BiFeO3 phase into a thin film nanocomposite structure, and the detailed connectivity scheme and coupling mechanism in terms of the microscopic origin of the measured high ME coefficients αME up to 21 V/(cm Oe) have not been clear up to now. The BaTiO3 -BiFeO3 superlattice heterostructures with clear spatial separation of both phases as published in [11,22,23] are mentioned in the discussion only for a comparison
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