Crystal and Magnetic Structure in Co-Substituted BiFeO₃

Ultra-high-resolution neutron diffraction studies of BiFe_0.8Co_0.2O₃ show a transition from a cycloidal space modulated spin structure at T = 10 K to a collinear G-type antiferromagnetic structure at T = 120 K. The model of antiparallel directions of Fe³⁺ and Co³⁺ magnetic moments at the shared Wyckoff position describes well the observed neutron diffraction intensities. On heating above RT, the crystal structure of BiFe_0.8Co_0.2O₃ changes from a rhombohedral R3c to a monoclinic Cm. At 573 K only the Cm phase is present. The collinear C-type antiferromagnetic structure is present in the Cm phase of BiFe_0.8Co_0.2O₃ at RT after annealing.