Exciton Absorption Lines in Antiferromagnetic Rare-Earth Orthochromites –With Particular Reference to YCrO₃–

A method of analyzing absorption spectra of Frenkel excitons associated with single-Cr 3+ transitions in antiferromagnetic rare-earth orthochromites is presented for both cases with and without an external magnetic field. As the simplest example, exciton lines of YCrO 3 in the spectral region of ∼13,800 cm -1 are studied in detail both experimentally and theoretically. Four magnetic-dipole lines are assigned to Davydov-split exciton lines associated with one single-Cr 3+ excitation, and matrix elements of excitation transfer in various Cr 3+ -pairs are determined. It is considered that spin-forbidden excitation transfer between the sublattices of opposite spins is released by a small canting of spins not responsible for the weak ferromagnetism. When a magnetic field along the crystal a -axis is increased up to 50 kOe, spin reorientation from the a -axis to the c -axis is observed in the change of intensities and polarizations of the exciton lines.