X-ray MCD Measurements in a high Tc molecular-based magnet

The molecular based magnet Cs^(I)[Ni^(II)Cr^(III)(CN)6]. 2H2O is a ferromagnet with a Curie temperature Tc = 90 K. Its structure consists of face centred cubic lattice of Ni^(II) ions connected by Cr(CN)6 entities. We have recorded X-ray Magnetic Circular Dichroism (XMCD) at nickel L2,3 edges. It clearly evidences that nickel II is in a high spin configuration and ferromagnetically coupled to the surrounding Cr^(III). Through Crystal Field Multiplet calculations, we have determined the precise ground state of 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 rulesderived for XMCD, it has been possible to extract the orbital and spin contributions to the total magnetic moment. A too small magnetic moment is found in nickel. A complete calculation taking into account the multiplet coupling effect and the covalent hybridisation showed that hybridisation cannot be responsible for the experimental low nickel magnetic moment. The origin of this effect is discussed.