Field-induced Polar Order at the N\'eel 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 microC/cm2) below the N'eel temperature of chromium (TN Cr) in weakly ferromagnetic rareearth orthochromites, RCrO3 (R=rareearth) but only when the rareearth ion is magnetic. Intriguingly, the polarization in ErCrO3 (TC ~ 133 K) disappears at a spin reorientation (Morin) transition (TSR ~ 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 applied magnetic field, indicating a strong magneto electric effect. We suggest that the polar order occurs in RCrO3, due to the combined effect of poling field that breaks the symmetry and the exchange field on R ion from Cr sublattice stabilizes the polar state. We propose that a similar mechanism could work in the isostructural rareearth orthoferrites, RFeO3 as well.