Size Effect on Local Magnetic Moments in Ferrimagnetic Molecular Complexes: An XMCD Investigation

Molecular chemistry allows to synthesize new magnetic systems with controlled properties such as size, magnetization or anisotropy. The theoretical study of the magnetic properties of small molecules (from 2 to 10 metallic cations per molecule) predicts that the magnetization at saturation of each ion does not reach the expected value for uncoupled ions when the magnetic interaction is antiferromagnetic. The quantum origin of this effect is due to the linear combination of several spin states building the wave function of the ground state and clusters of finite size and of finite spin value exhibit this property. When single crystals are available, spin densities on each atom can be experimentally given by Polarized Neutron Diffraction (PND) experiments. In the case of bimetallic MnCu powdered samples, we will show that X-ray Magnetic Circular Dichroism (XMCD) spectroscopy can be used to follow the evolution of the spin distribution on the MnII and CuII sites when passing from a dinuclear MnCu unit to a one dimensional (MnCu) _n compound.