Microscopic theory of magnetic interactions in KCuF3 and LaMnO3 - The role of charge transfer

We analyze the spin exchange interactions Jab and Jc in the A -type antiferromagnetic phase of KCuF3 and LaMnO3. One finds that the quasi one-dimensional spin interactions in KCuF3 are consistent with the charge-transfer type of insulating state, while the modifications of Jab and Jc by the charge-transfer superexchange terms are moderate in the Mott insulator LaMnO3. We point out on the example of LaMnO3 that a reliable determination of the experimental microscopic parameters of spin-orbital models is possible only when results of magnetic and optical experiments can be combined. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)