Magnetic Symmetry of Rare-Earth Orthochromites and Orthoferrites

A new magnetic symmetry of the rare-earth orthochromites and orthoferrites (RMO₃) below the second Néel temperature due to the reordering of the rare-earth spins is proposed. It is shown that the magnetic symmetry group is lowered to the subgroup of index two, thereby keeping the unit cell invariant. The proposed symmetry is shown to be consistent with the previously observed spin configuration of RMO₃ such as GdCrO₃, DyFeO₃, and DyCrO₃, and would also be consistent with those of other RMO₃. Although symmetry arguments and the calculation of the free energy show that two spin configurations are stable, in GdCrO₃, a future measurement of the magnetoelectric tensor would determine which configuration GdCrO₃ has. It is further predicted that the magnetoelectric effect should be observed in any RMO₃ below the second Néel temperature.