Epitaxial growth of Ni0.5Zn0.5Fe2O4+BiFeO3 composite films on SrTiO3 substrates

Ni0.5Zn0.5Fe2O4 (NZFO) + BiFeO3 (BFO) composite films were co-sputtered on (001) SrTiO3 (STO) substrates, which were confirmed to be epitaxial by both 1D and 2D X-ray diffractions. Conductive LaNiO3 (LNO) was sputtered on STO as bottom electrode for magnetoelectric (ME) measurement across film. The LNO layer was epitaxial and composed of nanocolumns perpendicular to substrate, which are helpful in reducing “substrate clamp” effect because of the flexibility of nanocolumns. NZFO + BFO films co-sputtered on LNO/STO were also epitaxial but both out-of-plane and in-plane textures of BFO were degraded with full width at half maximum of 1.974° and 2.55°, respectively, compared to 1.163° and 1.71° in films grown on bare STO. There were apparent tetragonal distortions in NZFO and BFO due to compressive in-plane strains imposed by epitaxial growth of larger NZFO and BFO lattices on smaller LNO lattice. Transmission electron microscopy (TEM) showed that the composite films were compact with clear boundaries between the NZFO and BFO phases, which were uniformly distributed with the ratio of about 35% NZFO and 65% BFO. Cross-sectional TEM revealed that crystallographic planes were very well aligned between BFO and LNO, but NZFO planes tilted 7.5° due to large lattice misfit. When measured vertically from LNO to top surface of composite film, large ME voltage coefficient at zero bias field was observed, which was 911 mVcm−1Oe−1 at the frequency of 8 kHz. This was ascribed to the large heteroepitaxial strains in composite films.