Magnetic Symmetry of Rare-Earth Orthochromites and Orthoferrites

TLDR

The study proposes a new magnetic symmetry for rare‑earth orthochromites and orthoferrites below the second Néel temperature arising from rare‑earth spin reordering. The authors use symmetry arguments and free‑energy calculations to show two stable spin configurations, suggesting that measuring the magnetoelectric tensor in GdCrO₃ will identify the actual configuration. The symmetry group is reduced to an index‑two subgroup while preserving the unit cell, matches known spin configurations of several RM O₃ compounds, and predicts a magnetoelectric effect in all RM O₃ below the second Néel temperature.

Abstract

A new magnetic symmetry of the rare-earth orthochromites and orthoferrites ($\mathrm{RM}{\mathrm{O}}_{3}$) below the second N\'eel 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 $\mathrm{RM}{\mathrm{O}}_{3}$ such as GdCr${\mathrm{O}}_{3}$, DyFe${\mathrm{O}}_{3}$, and DyCr${\mathrm{O}}_{3}$, and would also be consistent with those of other $\mathrm{RM}{\mathrm{O}}_{3}$. Although symmetry arguments and the calculation of the free energy show that two spin configurations are stable, in GdCr${\mathrm{O}}_{3}$, a future measurement of the magnetoelectric tensor would determine which configuration GdCr${\mathrm{O}}_{3}$ has. It is further predicted that the magnetoelectric effect should be observed in any $\mathrm{RM}{\mathrm{O}}_{3}$ below the second N\'eel temperature.