Patrick M. Woodward
Department of Chemistry and Biochemistry, The Ohio State University
Over the past several years we have been synthesizing and studying the magnetic properties of A2MOsO6 and A2MReO6 (Mg, Zn, Cr, Fe, Co, Ni) double perovskites in a quest to understand how the sign and strength of the superexchange interactions change as a function of the relative filling of the 3d and 5d orbitals, as well as the geometry of the crystal structure. In double perovskites where the 5d ion is the only magnetic ion we find that spin-orbit coupling plays a role, even for electron counts like 5d3 where it was assumed to be quenched. Furthermore, the sign and strength of the superexchange coupling between 5d ions is very sensitive to changes in the filling of the 5d orbitals and distortions of the crystal structure. In double perovskites that contain both 3d and 5d ions, differences in the energy levels of the 3d and 5d orbitals causes the 3d−5d superexchange coupling through the eg orbitals to be insignificant, thereby allowing longer range 5d−5d and 3d−3d superexchange coupling to be competitive. The strength of the competing superexchange interactions can be tuned by changing the filling of d-orbitals and is highly sensitive to changes in chemical pressure. Using these tools we can access magnetic states ranging from ferrimagnetic to antiferromagnetic to spin glasses. The findings of this work have important implications for efforts to design and understand magnetic materials for the 21st century.