100-nm-sized magnetic domain reversal by the magneto-electric effect in self-assembled BiFeO₃/CoFe₂O₄ bilayer films

A (001)-epitaxial-BiFeO₃/CoFe₂O₄ bilayer was grown by self-assembly on SrTiO₃ (100) substrates by just coating a mixture precursor solution. The thickness ratio of the bilayer could be controlled by adjusting the composition ratio. For example, BiFeO_x:CoFe₂O_x = 4:1 (namely Bi₄CoFe₆O_x) mixture solution could make a total thickness of 110nm divided into 85-nm-thick BiFeO₃ and 25-nm-thick CoFe₂O₄. Self-assembly of the bilayer occurred because the perovskite BiFeO₃ better matched the lattice constant (misfit approximately 1%) and crystal symmetry of the perovskite SrTiO₃ than the spinel CoFe₂O₄ (misfit approximately 7%). The magnetic domains of the hard magnet CoFe₂O₄ were switched by the polarization change of BiFeO₃ due to an applied vertical voltage, and the switched magnetic domain size was approximately 100nm in diameter. These results suggest that self-assembled BiFeO₃/CoFe₂O₄ bilayers are interesting in voltage driven nonvolatile memory with a low manufacturing cost.