The phase transition and superior multiferroic properties of (Mn, Co) co-doped BiFeO₃/CoFe₂O₄ double-layer films

(Mn, Co) co-doped BiFeO₃/CoFe₂O₄ (BFMCO/CFO) double-layer films were prepared on fluorine doped tin oxide (FTO)/glass substrates by the chemical solution deposition method. The effect of (Mn, Co) co-doping on crystal structure, ferroelectric and magnetic properties of BFO/CFO double-layer films was investigated. Mn and Co co-doping on the BiFeO₃ layer induced the tendency of phase transformation from rhombohedral to tetragonal. Well saturated polarization hysteresis loops were observed in BFMCO/CFO films, meanwhile a giant remanent polarization of the double-layer films (P_r ∼ 123 μC/cm², after compensation) is obtained. Furthermore, an enhanced ferromagnetic behavior of M_s ∼ 113 emu/cm³, M_r ∼ 67 emu/cm³ and H_c ∼ 1200 Oe is observed in BFMCO/CFO films. The reduction of Fe²⁺ and the existence of high valence iron ions with co-doping Mn and Co were confirmed by XPS. Therefore, the difference of magnetic moment of TM^2+/3+ and Fe³⁺ may play a more important role in the suppression of the spiral spin modulation. As a result, the introduction of (Mn, Co) and CoFe₂O₄ layer is an effective way to enhance the multiferroic behavior of the films.