100nm-Sized Magnetic Domain Reversal by Magneto-electric Effect in Self-assembled BiFeO3/CoFe2O4 Bilayer Films

Nature, Scientific Reports has published an article written by Keita Sone, Hiroshi Naganuma, Masaki Ito, Department of Applied Physics, Faculty of Science, Tokyo University of Science, 6-1-3 Niijuku, Katsushika, Tokyo 125-8585, Japan, Takamichi Miyazaki, Department of Instrumental Analysis, Tohoku University, 6-6-11 Aoba, Aramaki, Aoba, Sendai 980-8579, Japan, Takashi Nakajima, and Soichiro Okamura, Department of Applied Physics, Faculty of Science, Tokyo University of Science, 6-1-3 Niijuku, Katsushika, Tokyo 125-8585, Japan.

(a) Cross-sectional TEM image of a wide area of the sample,
(b)(e) electron-diffraction patterns taken from circles (i)(iv),
and cross-sectional high-resolution TEM images taken at the (f)
BiFeO₃/La-SrTiO₃ and (g) CoFe₂O₄/BiFeO₃ interfaces.
(h) Out-of-plane /2 XRD pattern, GI-XRD patterns around (i) (311)
and (j) (440) of the CoFe₂O₄ diffraction peaks, and RSMs around the (d) 004
and (e) 204 spots of the (001)-epitaxial-BiFeO₃/CoFe₂O₄ film.

Click to enlarge

Abstract: “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, a 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.“