X-ray magnetic circular dichroism spectra on the superparamagnetic transition metal-ion clusters [Mn12O12(CH3COO)16(H2O)24].2CH3COOH.4H2O (Mn12) and [Fe8O2(OH)12(tacn)6]Br8 9H2O (Fe8)

The first X-ray magnetic circular dichroism (XMCD) spectra on the title compounds are presented. In Mn12 the Mn(III) and Mn(IV) contributions are clearly apparent and evidence the opposite spin arrangement of the two ions. For Fe8 the typical two peak structure of Fe(III) compounds is found. Quantitative analysis yielded for both compounds a negligible <Lz>. This is the first direct experimental evidence of the quenching of the angular momentum in these systems. For Fe8 we found <Sz>=1.14 in mB units per Fe atom, in reasonable agreement with the S=10 ground state assumed for in this system.