New Paradigms in Heavy Transition Metal Molecular Magnetism

The spatially more extended d orbitals in heavy transition metals (i.e. 4d and 5d) usually lead to stronger inter-site hopping effects, which could dominate in 4d/5d transition metal cluster magnets. In that case, heavy transition metal molecular magnets can host a variety of intracluster interactions due to the delicate competition between Hund's coupling and orbital hopping that is enhanced by the larger spatial extent of the d-orbitals as compared to their 3d transition metal counterparts. Therefore, the electronic ground states of the molecules can be governed by local moment physics, molecular double exchange, metal-metal bonding, or even orbital selectivity. The variation in electronic ground states has been shown to generate new spin liquid candidates, low-lying spin state transitions, and large zero field splitting that may give rise to single molecule magnets with incredibly high blocking temperatures. This talk will present my recent work in this novel area of heavy transition metal molecular magnets. Topics covered will include experimental and theoretical criteria for establishing their electronic ground states, orbital selectivity at the molecular level, and the exotic collective magnetic properties that can be realized in these materials. We have carried out neutron spectroscopic and diffraction studies, dc and ac magnetic susceptibility, specific heat, and μSR measurements. We synthesized and systematically studied the magnetic properties of the 6H-perovskite ruthenates Ba₃MRu₂O₉ (M = Ca, Y, In, Lu, La, Ce) featuring ruthenium dimers, and a few molybdenum trimer based molecular magnets with [Mo₃]¹¹⁺ building blocks. These two systems are among the most common structures of heavy transition metal molecular magnets.