Probing local magnetization in heterometallic molecular nanomagnets by XMCD.

We developed a method to probe the local electronic and magnetic features within molecular spin clusters by using X-ray Absorption Spectroscopy (XAS) and Magnetic Circular Dichroism (XMCD). Thanks to their high sensitivity and selectivity, we separately investigate local symmetries, electronic configuration, orbital and spin magnetic moments of the magnetic ions. The key idea is to consider heterometallic spin clusters with a single extra ion and to exploit the chemical selectivity of XMCD in order to resolve magnetic features within spin cluster and, in particular, of the single extra-ion. We focused on two ad-hoc molecular systems. The first one is a family of antiferromagnetic (AF) spin segments namely Cr6InNi and Cr7InNi, which are odd- and even membered spin cyclic systems magnetically broken by an In3+ ion [1]. We show that the dichroic signal taken at the Ni L2,3 edges is particularly sensitive to the topology or nuclearity of the cyclic system, evidencing that, in magnetically broken rings, the spin of nickel tends to be aligned along the magnetic-field direction. Secondly we have studied heterometallic oxocentered carboxylate-bridged Cr2Cu trimers (see Fig. 1). We found that, in presence of an applied magnetic field, the projection of the Cu spin moment is opposite to that of Cr and we derived the separate contributions of Cu2+ and Cr3+ to the total magnetic moment of Cr2Cu in agreement with spin-Hamiltonian calculations [2]. Discussion on both these cases allows us to generalize this method to other molecular spin systems.