Stress induced linear dichroism of antiferromagnetic perovskites : KNiF 3, KCoF3 and RbCoF3. II. —Spectroscopic calculations and assignments

We have demonstrated in the preceding article that the linear dichroism (LD) under stress in magnetically ordered compounds has a magnetic origin. Here we present the calculation of this LD for d-d transitions in KNiF 3, KCoF3, RbCoF3. We compare the corresponding LD spectra to the magnetic linear dichroism (MLD) spectra of diluted compounds : Ni2+/KZnF3, Co2+/KMgF3. In KNiF3, the LD under a < 001 > stress, below TN, is the LD given by a domain whose spins lie along the < 001 > direction. We have calculated it for exciton and exciton-phonon bands by applying the formula of the MLD for paramagnetic compounds to stressed magnetic crystals. The transitions of Co2+/KMgF3 give no MLD. The LD under stress in KCoF3 for T < TN is related to the fact that the exchange and the spin-orbit interactions are of the same order of magnitude. The agreement between our calculations and the experimental spectra allows us to assign several spectral terms and spin-orbit components in KNiF3. We can also conclude that our results are consistent with two hypotheses we made about the magnetic materials : the k = 0 phonons are the most efficient in the absorption spectra and the exchange interaction in the excited states is weak.