Magnetization Reversal in BiFe_0.9Co_0.1O₃ - 2:
Electric Field induced Magnetic Domain Change after Ferroelectric Topological Domain Switching in BiFe_0.9Co_0.1O₃ nanodots

Multiferroic BiFeO₃ (BFO) is a room-temperature ferroelectric () and G-type antiferromagnet () with long range spin cycloidal modulation of 63 nm. For its strong magnetoelectric coupling arising from the Dzyaloshinskii-Moriya interaction coupled with the octahedral tilting, electric field control of antiferromagnetic order and magnetic signal reading through spin valve junction with ferromagnet heterostructure have been reported attracting application as an ultralow-power consumption spintronics or magnetic memory devices. In addition, ferroelectric topological domains such as vortex or center structures are also drawing attention for showing nonvolatile resistive switching behavior. Combination of ferroelectric and antiferromagnetic domains in BFO is extremely intriguing in terms of both fundamental science and device application.
Previously, our group discovered that by partial substitution of Co for Fe, spin canted collinear structure is stabilized by breaking the spin cycloid, inducing macroscopic weakly ferromagnetic moment () appeared perpendicular to the electric polarization. By fabricating (001)-oriented BiFe_0.9Co_0.1O₃ (BFCO) thin films by pulsed laser deposition (PLD) method, we observed both ferroelectric and ferromagnetic domains with striped pattern structures using combined piezoresponse and magnetic force microscopies (PFM and MFM). Moreover, by applying an electric field in the film normal direction, out-of-plane magnetization reversal was demonstrated following by polarization switching which enables electric field control of weakly ferromagnetic moment in a single phase. Furthermore, by transferring a porous anodic alumina oxide mask on Nb:SrTiO₃ (STO) (001) substrate, BFCO nanodots having ~ 190 nm diameter were deposited by PLD showing topological ferroelectric quadrant center domains and magnetic multidomain structures. However, the correlation between both domain structures was not clearly revealed due to its weak magnetic signal.
In this study, we performed highly sensitive scanning diamond nitrogen-vacancy center magnetometry (SNVM) to investigate the correlation of both ferroelectric and ferromagnetic domains and analyze the magnetic domain change after polarization switching in BFCO/Nb:STO (001) nanodots. As a result, magnetization reversal was demonstrated following by both out-of-plane and in-plane polarization switchings, distinct from thin films accompanying out-of-plane polarization switching without reconstruction of ferroelectric domains. This result highlights that single-phase ferroelectric and weakly ferromagnetic properties are preserved and electric-field-induced magnetization reversal in BFCO down to nanoscale region, which is promising for the realization of electric-field writing magnetic read-out memory devices.