High-spin Co3+ as a trigger of weak ferromagnetism in Co-substituted BiFeO3

Electric field control of magnetization is expected to be utilized for energy-efficient nonvolatile magnetic memory application. $\mathrm{C}{\mathrm{o}}^{3+}$-substituted $\mathrm{BiFe}{\mathrm{O}}_3$ (BFCO) exhibits both ferroelectric polarization ($\sim 100\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$) and the spin canting induced weakly ferromagnetic moment ($\sim 0.03{\mu }_B/\mathrm{f}.\mathrm{u}.$) perpendicular to each other at room temperature. Magnetization reversal by electric field was observed in a BFCO thin film. In our previous theoretical calculation, spin canted collinear structure having a weak ferromagnetic moment was stabilized when high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ was substituted in $\mathrm{BiFe}{\mathrm{O}}_3$. To confirm the presence of high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ in BFCO, the pressured-induced spin-state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ was investigated by first-principles calculations and Co $K\beta$ synchrotron x-ray emission spectroscopy. We investigated the spin state of $\mathrm{C}{\mathrm{o}}^{3+}$ during the pressure-induced structural transition from $R3c$ to Pnma. As a result, a pressure-induced transition from high-spin to low-spin $\mathrm{C}{\mathrm{o}}^{3+}$ was predicted. Moreover, the presence of high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ at ambient pressure was confirmed in $\mathrm{BiF}{\mathrm{e}}_{0.8}\mathrm{C}{\mathrm{o}}_{0.2}{\mathrm{O}}_3$ by observing the decrease of $K{\beta }^{\prime }$ peaks of Co $K\beta$ x-ray emission spectra under pressure. This study provides evidence that the weak ferromagnetism of BFCO is attributed to high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ and provides insight that may help to enhance weak magnetization by substitution of other elements.