Measurement of Magnetic Moment at the Atomic Scale in a High TC Molecular Based Magnet

The molecular-based magnet Cs I [Ni II Cr III (CN) 6 ]·2H 2 O is a ferromagnetic with a Curie temperature T C = 90 K. Its structure consists of face-centered cubic lattice of Ni II ions connected by Cr(CN) 6 entities. We have recorded X-ray magnetic circular dichroism (XMCD) at nickel L 2,3 edges. It clearly evidences that nickel(II) is in a high-spin configuration and ferromagnetically coupled to the surrounding Cr III . Through ligand field multiplet calculations, we have determined the total magnetic moment carried by Ni II . Special attention has been paid to the magnetic anisotropy that complicates the calculation of the cross section for a powder. By using sum rules derived for XMCD, it has been possible to extract the orbital and spin contributions to the total magnetic moment. A somewhat too small magnetic moment is found on nickel. A complete calculation taking into account the multiplet coupling effect and the covalent hybridization allowed to determine the precise ground state of nickel and showed that hybridization cannot be responsible for the experimental low nickel magnetic moment. The origin of this effect is discussed.