X-ray magnetic circular dichroism investigation of spin and orbital moments in Cr-8 and Cr7Ni antiferromagnetic rings

We investigated the electronic and magnetic properties of thick films of molecular {Cr-8} and {Cr7Ni} antiferromagnetic rings by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). We determined the local symmetries, the electronic configuration, and the values of orbital and spin moments at the Cr and Ni sites of the molecular rings. XMCD measurements show that the correlation between the Cr and Ni spins in the {Cr7Ni} molecular ring switches from antiferromagnetic to ferromagnetic with increasing temperature. Experimental data are interpreted using XMCD sum rules that allow the separate evaluation of the spin and the orbital contributions to the total magnetic moment of the ring as a function of temperature and magnetic field. The magnetic behaviors experimentally observed are compared with the results of spin-Hamiltonian calculations, based on microscopic parameters derived by inelastic-neutron scattering and low-temperature specific-heat measurements. The very good agreement between experimental data and calculations is a clear indication of the integrity of molecules. The temperature dependence of the ion magnetic moments results from the interplay between Zeeman and isotropic-exchange contributions, and is well captured by the theoretical model.