Exchange interaction in binuclear complexes with rare-earth and copper ions: A many-body model study

We have used a many-body model Hamiltonian to study the nature of the magnetic ground state of heterobinuclear complexes involving rare-earth and copper ions. We have taken into account all diagonal repulsions involving the rare-earth 4f and 5d orbitals and the copper 3d orbital. In addition, we have included direct exchange interaction, crystal field splitting of the rare-earth atomic levels and spin-orbit interaction in the 4f orbitals. We have identified the interorbital 4f repulsion U-ff and crystal field parameter Delta(f) as the key parameters involved in controlling the type of exchange interaction between the rare earth 4f and copper 3d spins. We have explored the nature of the ground state in the parameter space of U-ff, Delta(f), spin-orbit interaction strength lambda, and the 4f filling n(f). We find that these systems show low-spin or high-spin ground state depending on the filling of the 4f levels of the rare-earth ion and ground state spin is critically dependent on U-ff and Delta(f). In case of half filling [Gd(III)] we find a reentrant low-spin state as U-ff is increased, for small values of Delta(f), which explains the recently reported apparent anomalous antiferromagnetic behavior of Gd(III)-radical complexes. By varying U-ff we also observe a switch over in the ground state spin for other fillings . We have introduced a spin-orbit coupling scheme which goes beyond the L-S or j-j coupling scheme and we find that spin-orbit coupling does not significantly alter the basic picture