The Mechanism of Participation of Non‐Magnetic B‐Site Cations in the Superexchange in Spinels

Abstract The superexchange between magnetic cations at the A‐sites of spinels with non‐magnetic B‐site cations (e.g. CoAl 2 O 4 , Co 3 O 4 , CoRh 2 O 4 etc.) is analysed by using a four‐electron, four‐orbital model. The problem is treated in the framework of the CI‐method up to the fourth‐order of the perturbation theory. In the case of Co(III) and Rh(III) in the low‐spin state as B‐site cations four contributions are found: two negative and two positive. One of the ferromagnetic terms is not connected with Hund's rule but has the same origin (delocalization) as the antiferromagnetic ones. The resulting superexchange is always antiferromagnetic. When B is Al 3+ there is only one small antiferromagnetic term.