The Magnetism of Cubic and Hexagonal Perovskites Containing 4d and 5d Transition Metal Ions

The magnetism of oxides containing transition metal ions with partially filled 4d or 5d orbitals differ from their 3d counterparts in several fundamental ways. Firstly, spin-orbit coupling is much stronger, which can lead to entanglement of spin and orbital degrees of freedom. Secondly, the 5d orbitals extend further from the nucleus which leads to considerable hybridization with the 2p orbitals of the surrounding oxide ions or in hexagonal perovskites direct overlap with the d-orbitals of neighboring transition metals to form metal-metal bonds. The combination of the two effects leads to competing magnetic ground states are very sensitive to distortions of the lattice. In the first part of the talk, I will discuss the magnetism of double perovskites containing ions with a 5d¹ configuration. In those compounds where the cubic symmetry is favored by the constituent ions, Coulomb interactions between orbitals on neighboring ions leads to orbital ordering that precedes and facilitates spin ordering. The absence of magnetic reflections in high intensity neutron diffraction studies suggests ordering of multipolar magnetic moments. In the second half of the talk I compare and contrast the magnetism and potential metal-metal bonding of hexagonal perovskites containing 3d transition metal ions like Cr with those containing 4d transition metal ions like Ru. Strong overlap of the 4d orbitals in the latter case results in magnetic moments that are spread over the entire cluster of face-sharing octahedra rather than localized on an individual transition metal ion. The triangular network of Ru₃O₁₂ clusters results in frustration that is promising for realization new spin liquids, provided the exchange interactions between layers can be minimized.