$\mathrm{SmFe}{\mathrm{O}}_3$, a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric. However, we have found that $\mathrm{SmFe}{\mathrm{O}}_3$ is surprisingly ferroelectric at room temperature with a small polarization along the $b$ axis of $Pbnm$. First-principles calculations indicate that the canted antiferromagnetic ordering with two nonequivalent spin pairs is responsible for this extraordinary polarization and that the reverse Dzyaloshinskii-Moriya interaction dominates over the exchange-striction mechanism in the manifestation of the improper ferroelectricity. $\mathrm{SmFe}{\mathrm{O}}_3$ further exhibits an interesting phenomenon of spontaneous magnetization reversal at cryogenic temperatures. This reversal is attributed to the activation of the Sm-spin moment which is antiparallel to the Fe-spin moment below $\sim 5\mathrm{K}$.

• Received 11 May 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.117201