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 {~} 0.2--0.8 {μ}C/cm²) below the N\'eel temperature of chromium (TN{}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 (${T}SR{~}22$ 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 RCrO₃, 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 RFeO₃ as well.