Non-collinear magnetic structures of TbCoO₃ and DyCoO ₃

The orthoperovskites TbCoO₃ and DyCoO₃ with Co ³⁺ in a non-magnetic low-spin state have been investigated by neutron diffraction down to 0.25 K. Magnetic ordering is evidenced below T_N = 3.3 K and 3.6 K, respectively, and the ordered arrangements are of canted type, A_xG_y for TbCoO₃ and G_xA _y for DyCoO₃ in Bertaut's notation. The experiments are confronted with the first-principle calculations of the crystal field and magnetism of Tb³⁺ and Dy³⁺ ions, located in the Pbnm structure on sites of C_s point symmetry. Both these ions exhibit an Ising behavior, which originates in the lowest energy levels, in particular in accidental doublet of non-Kramers Tb³⁺ (4f⁸ configuration) and in ground Kramers doublet of Dy³⁺ (4f⁹) and it is the actual reason for the non-collinear AFM structures. Very good agreement between the experiment and theory is found. For comparison, calculations of the crystal field and magnetism for other systems with Kramers ions, NdCoO₃ and SmCoO₃, are also included.