Weak ferromagnetism and spin reorientation in antiferroelectric BiCrO3

iCrO3 is an antiferroelectric perovskite known to exhibit an unconventional spin reorientation transition between antiferromagnetic structures, accompanied by a large jump in weak ferromagnetism. Using a combination of neutron powder diffraction, magnetometry, and symmetry analysis, we confirm the dominant G-type antiferromagnetic order below TN=111 K and identify the magnetic phase transition with a spontaneous rotation of Cr3+ moments from the b axis to a particular direction in the ac plane. We demonstrate the role of antiferroelectric displacements produced by the Bi3+ lone-pair electrons and octahedral rotations in establishing spin canting via the antisymmetric Dzyaloshinskii-Moriya interaction. This mechanism results in weak ferromagnetism above and below the spin-reorientation and explains the dramatic increase in net magnetization on cooling.