Field-induced antiferromagnetic cone structure in multiferroic BiFeO₃

BiFeO₃, a rare multiferroic compound that shows antiferromagnetism and ferroelectricity simultaneously above room temperature (T_N~640 K), has been studied intensively. Recently, a new magnetic phase has been found between the cycloidal and canted antiferromagnetic phase in magnetic field [1]. We performed neutron diffraction measurements in high static magnetic fields up to 19 T on HFM/EXED facility at BER II research reactor in Helmholtz Zentrum Berlin. We successfully observed weak incommensurate peaks around 13 T which split along the magnetic field direction [1,1,-2] at the (1/2, 1/2, 1/2) main peak. The magnetic field and temperature region, where the incommensurate peaks appear, is consistent with that in the phase diagram obtained by the magnetization measurements [1]. We confirmed that the magnetic structure in the intermediate phase is the antiferromagnetic cone structure which is predicted theoretically. We will show the detailed magnetic structure (magnetic wave vector and cone angle) and discuss the relation between the magnetic structure and the magnetoelectric effect. [1] S. Kawachi et al., Phys. Rev. Materials 1, 024408 (2017).