Soft-x-ray magnetic circular dichroism on a paramagnetic two-iron system

The sign of the x-ray magnetic circular dichroism signal has in general been used to determine the magnetic coupling between ions with different oxidation states or different elements. We have investigated the coupling between two distorted-tetrahedral-coordinated irons with two different oxidation states in a dinuclear cluster. The two iron atoms are antiferromagnetically coupled and because of a strong chemical shift between the two oxidation states one would expect to see an up-down effect in the magnetic circular dichroism signal at the iron ${\mathit{L}}_3$ edge. We observe an only positive effect, which is inconsistent with a strong antiferromagnetic coupling. We can explain the results by taking the individual zero-field splittings of both iron atoms into account. This study uses magnetic circular dichroism in the soft-x-ray range to study the magnetic coupling between transition-metal sites in paramagnetic clusters, and although still limited by low count rates it shows the potential and subtleties of this technique. © 1996 The American Physical Society.