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BiFeO 3 and BaTiO 3 were used to grow homogeneous composite thin films and multilayer heterostructures with 15 double layers by pulsed laser deposition. The perpendicular strain of the films was tuned by employing different substrate materials, i.e. SrTiO 3 (0 0 1), MgO(0 0 1) and MgAl 2 O 4 (0 0 1). Multiferroic properties have been measured in a temperature range from room temperature down to 2 K. The composite films show a high ferroelectric saturation polarization of more than 70 µC cm −2 . The multilayers show the highest magnetization of 2.3 emu cm −3 , due to interface magnetic moments and exchange coupling of the included weak ferromagnetic phases. The magnetoelectric coupling of the BaTiO 3 -BiFeO 3 films was investigated by two methods. While the ferroelectric hysteresis loops in magnetic fields up to 8 T show only minor changes, a direct longitudinal AC method yields a magnetoelectric coefficient α ME = ∂E/∂H of 20.75 V cm −1 Oe −1 with a low µ 0 H DC of 0.25 T for the 67% BaTiO 3 -33% BiFeO 3 composite film at 300 K. This value is close to the highest reported in the literature.

Multiferroic (BaTiO3-BiFeO3) × 15 multilayer heterostructures show high magnetoelectric (ME) coefficients αME up to 24 V/cm·Oe at 300 K. This value is much higher than that of a single-phase BiFeO3 reference film (αME = 4.2 V/cm·Oe). We found clear correlation of ME coefficients with increasing oxygen partial pressure during growth. ME coupling is highest for lower density of oxygen vacancy-related defects. Detailed scanning transmission electron microscopy and selected area electron diffraction microstructural investigations at 300 K revealed antiphase rotations of the oxygen octahedra in the BaTiO3 single layers, which are an additional correlated defect structure of the multilayers.

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