Magnetic properties of an antiferromagnetic orthoferrite

The orthoferrite DyFeO₃, weakly ferromagnetic at room temperature, was found to become antiferromagnetic at 36°K. Symmetry considerations suggest that at this transition point the iron spin-axis rotates out of the plane perpendicular to the Moriya-Dzyaloshinsky vector, becoming parallel to it. On this basis the magnetic point group mmm is assigned to DyFeO₃ below the transition. Further evidence for the correctness of this assignment has been obtained by the observation of a linear dependence of the susceptibility on the magnetic field χ_xy=CH_z, and cyclic permutations as allowed by the mmm symmetry. Another transition, which we associate with an antiferromagnetic ordering of the Dy³⁺ moment, is found at T_N=3.7°K. The magnetization curves measured below 4.2°K at fields up to 40 kG are interpretable, to a good approximation, in terms of |±15/2〉 Kramers' doublet whose axis lies in the mirror plane of the crystal, making an angle of approximately 30°with the [010] axis. Below T_N metamagnetic transitions were observed. Symmetry arguments show that provided the chemical and magnetic unit cells are identical, the magneto-electric effect is allowed in DyFeO₃ below T _N.