Magnetization Reversal in BiFe_0.9Co_0.1O₃ Thin Films 2 - Control of Ferroelectric and Ferromagnetic Domains by Trailing Fields

BiFeO₃ (BFO) is the most widely investigated multiferroic material with a large electric polarization along the [111] of pseudocubic cell with a ferroelectric Curie temperature of 1103 K and an antiferromagnetic Néel temperature of 643 K. The presence of a cycloidal spin modulation prohibits the appearance of spontaneous magnetization. Partial Co substitution for Fe changes the spin structure to a collinear one with a spontaneous magnetization owing to the spin canting preserving the large electric polarization [1]. Therefore, BiFe_0.9Co_0.1O₃ (BFCO) is a multiferroic material with both a large electric polarization and a spontaneous magnetization at room temperature, which are perpendicular to each other. The magnetization reversal accompanying 71° polarization switching in a thin film has been observed at room temperature, which is expected to be utilized in ultra-low power consumption magnetic memory devices [2]. However, it is necessary to understand the behaviors of ferroelectric and magnetic domains during the polarization reversal process and deterministically control the polarization and magnetization direction.<br/>In this study, the polarization of BFCO grown in (001) orientation was controlled in both out-of-plane and in-plane direction by utilizing the effective in-plane electric field, so-called trailing field, resulting from the scanning of the cantilever [3]. The polarization of the as-grown film was limited to four out-of-plane downward directions by self-poling effect. The cantilever was scanned with a bias voltage of −7 V for out-of-plane polarization switching, which resulted in a striped ferroelectric domain with two types of out-of-plane upward polarization affected by the trailing field. Then, the same surface location was scanned with the cantilever, the slow scan direction of which is opposite to the in-plane component of the polarization, with an increased tip bias (−10 V). This process corresponded to increasing an effective in-plane electric field without changing the direction of the out-of-plane electric field. After such in-plane electric field application, the ferroelectric domains were observed by piezoresponse force microscopy (PFM). It was found that all the polarizations were oriented in the direction of the trailing field while maintaining the striped domain. The magnetic domains were also observed by magnetic force microscopy (MFM) and was found to have a striped domain shape similar to that of the ferroelectric domain. From these results, we conclude that in-plane polarization can be controlled by using the trailing field. The magnitude of the electric trailing field generated by −10 V bias voltage was estimated to be about 40 kV/cm⁻¹. Normally, about 200 kV/cm⁻¹ is required for in-plane polarization reversal of BiFeO₃ using planar electrodes, but in this trailing field, in-plane polarization reversal was achieved with only one-fifth of that voltage.<br/>Consequently, we succeeded in changing the polarization direction to any desired direction by controlling three main factors: the sign and magnitude of the voltage between the cantilever and the bottom electrode, and the slow scan direction (the direction of electric trailing field). Magnetic domain imaging by MFM revealed that the ferroelectric and magnetic domains have similar shapes, suggesting a strong correlation between polarization and magnetism, indicating the magnetic domain can also be controlled by the trailing field. The above results suggest that the electric trailing field is a valuable tool to manipulate the ferroelectric and magnetic domains in multiferroic BFCO thin films.<br/>[1] H. Hojo et al., Adv. Mater. 29, 1603131 (2017).<br/>[2] K. Shimizu et al., Nano Lett. 19, 1767 (2019).<br/>[3] T. Itoh et al., Appl. Phys. Express 15, 023002 (2022).