Abstract of Publication No. 278
Expressions in the early literature for kinetic exchange in magnetically coupled
systems are critically analyzed by second-order perturbation theory and
substituted by corrected versions. The widely held belief that kinetic exchange
is always antiferromagnetic is found to be incorrect. Ferromagnetic kinetic
exchange terms are found to be more important than hitherto assumed. The
quantity I /U, where I stands for intraatomic
exchange interactions and U is the energy difference between the ground
electron configuration and a charge-transfer configuration, plays a crucial role
in the competition between ferromagnetic and antiferromagnetic contributions.
I /U readily exceeds the value 1/5
considered as an upper limit by Anderson. I is found to be
proportional to the number of unpaired electrons on a given magnetic center.