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. Co³⁺-substituted BiFeO₃ (BFCO) exhibits both ferroelectric polarization (~1004pt0exC/cm²) and the spin canting induced weakly ferromagnetic moment (~0.034pt0exB/4pt0exf.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 Co³⁺ was substituted in BiFeO₃. To confirm the presence of high-spin Co³⁺ in BFCO, the pressured-induced spin-state transition of Co³⁺ was investigated by first-principles calculations and Co Kβ synchrotron x-ray emission spectroscopy. We investigated the spin state of Co³⁺ during the pressure-induced structural transition from $R3c$ to Pnma. As a result, a pressure-induced transition from high-spin to low-spin Co³⁺ was predicted. Moreover, the presence of high-spin Co³⁺ at ambient pressure was confirmed in BiFe0.8Co0.2O₃ by observing the decrease of Kβ^' peaks of Co Kβ x-ray emission spectra under pressure. This study provides evidence that the weak ferromagnetism of BFCO is attributed to high-spin Co³⁺ and provides insight that may help to enhance weak magnetization by substitution of other elements.