Multiferroic order parameters in rhombic antiferromagnets RCrO3.

Currently, active research is focused on multiferroic rare-earth orthochromites (RCrO3), due to various structural instabilities and couplings in these materials understanding the underlying magnetoelectric mechanisms is a challenge. In this paper, we explore magnetoelectric properties of RCrO3 in the framework of symmetry analysis. We demonstrate the presence of electric dipole moments localized in the vicinity of Cr3+ ions in RCrO3. The inversion symmetry breaks due to the displacements of oxygen ions from their highly symmetric positions in the parent perovskite phase. Our calculations show that electric dipoles are arranged in an antiferroelectric mode. In consistence with the symmetry of RCrO3 we introduce the basic distortive order parameters (OP) (D, Q2, Q3, P - polar OP and - axial OP) and classify them according to the irreducible representations of the RCrO3 symmetry group (D2h16). We determine the symmetry - allowed couplings between distortive, ferroelectric and magnetic orderings and find the possible exchange- coupled magnetic and ferroelectric structures. The presented analysis makes it possible to explain experimentally observed polarization reversal and the concomitant reorientation of spins in a series of RCrO3 compounds and to predict the possible scenarios of phase transitions in RCrO3.