Field-induced polar order at the Néel temperature of chromium in rare-earth orthochromites: Interplay of rare-earth and Cr magnetism

We report field-induced switchable polarization ($P$ \ensuremath{\sim} 0.2--0.8 \ensuremath{\mu}C/cm${}^{2}$) below the N\'eel temperature of chromium (${T}_{N}$${}^{\mathrm{Cr}}$) in weakly ferromagnetic rare-earth orthochromites, $R$CrO${}_{3}$ ($R$ $=$ rare earth) but only when the rare-earth ion is magnetic. Intriguingly, the polarization in ErCrO${}_{3}$ (${T}_{C}$ $=$ 133 K) disappears at a spin-reorientation (Morin) transition (${T}_{SR}\ensuremath{\sim}22$ K) below which the weak ferromagnetism associated with the Cr sublattice also disappears, demonstrating the crucial role of weak ferromagnetism in inducing the polar order. Further, the polarization ($P$) is strongly influenced by an applied magnetic field, indicating a strong magnetoelectric effect. We suggest that the polar order occurs in $R$CrO${}_{3}$, due to the combined effect of the poling field that breaks the symmetry and the exchange field on the R ion from the Cr sublattice that stabilizes the polar state. We propose that a similar mechanism could work in the isostructural rare-earth orthoferrites $R$FeO${}_{3}$ as well.