Magnetization Reversal in BiFe_0.9Co_0.1O₃ I: Magnetic Domain Change Induced by In-Plane Electric Polarization Switching in BiFe_0.9Co_0.1O₃ Thin Film

Perovskite bismuth ferrite (BiFeO₃) is a multiferroic material of practical interest because of the coexistence of ferroelectricity and antiferromagnetism at room temperature. If the polarization reversal induced by an electric field accompanies magnetization reversal of BiFeO₃, such a phenomenon can be utilized for an ultra-low-consumption voltage-write magnetic-read-out memory device. However, BiFeO₃ lacks net magnetization due to the presence of a long-range cycloidal modulation, which hampers straightforward magnetic actuation. Co-substitution in BiFeO₃ (BiFe_0.9Co_0.1O₃; BFCO) destabilizes the cycloidal modulation of spins and generates a canted collinear spin state with non-zero saturation magnetization. In addition, a magnetic easy plane perpendicular to electric polarization has been verified in the single-crystalline bulk BFCO and BFCO thin film on SrTiO₃(111). Recently, we reported domain observation of piezoresponse and magnetic force microscopies (PFM and MFM) in BFCO on GdScO₃(110)_o. We found that polarization switching without the reconstruction of the striped domain structure was necessary for that out-of-plane magnetization reversal by applying an electric field.
In this study, we focused on the magnetic domain change induced by applying an in-plane electric field via a patterned Pt electrode, because in-plane and out-of-plane polarization switchings likely lead to different magnetization directions after the reversal, given that the inversion of the Dzyaloshinskii−Moriya (DM) vector of BiFeO₃ depends on the switching path of polarization switching.
BFCO epitaxial thin films were grown on GdScO₃(110) substrate by using pulsed laser deposition. patterned Pt electrodes were prepared with the dc magnetron sputter via the standard lift-off process. Atomic force microscope (AFM) and PFM confirmed step-terrace surface morphology and striped ferroelectric domains, respectively. After applying 22.5 V between the Pt electrodes, both the ferroelectric and magnetic domains change, but their boundaries are closely matched, indicating that the ferroelectric and magnetic domains are correlated with each other. However, even though the ferroelectric and ferromagnetic domains preserved striped ferroelectric domain structures, the MFM contrast was completely preserved where in-plane 71 degree polarization switching occurred. These experiments have demonstrated that the occurrence or absence of magnetization reversal is deterministic and depends on the direction of the applied electric field. These findings facilitate the design of memory devices utilizing BFCOs and other multiferroic materials.