Frustrated Magnetism in a Potential Quantum Material Based on Spin-1/2 Co²⁺ Dimers

The layered-triangular-based compound Rb₂Co₂(SeO₃)₃ was designed and synthesized to investigate frustrated magnetism in a Co²⁺-based system. The crystal structure consists of a planar triangular array of face-sharing Co²⁺O₆ octahedra forming dimers separated by Rb⁺ layers. Comparison of Rb₂Co₂(SeO₃)₃ to the isostructural known compound K₂Co₂(SeO₃)₃ suggests that the larger-size Rb⁺ spacers weaken its interlayer interactions. The magnetic entropy reveals the effective S = 1/2 nature of Co²⁺ at low temperatures in this compound. A feature at around 3.4 K appears in both the magnetic data and heat capacity data in zero field, which can be sensitively tuned by applying an out-of-plane magnetic field, and it shows a substantially reduced heat capacity change than expected for a 3D magnetic ordering, suggesting competing magnetic interactions and a frustrated nature. Successive magnetization plateaus observed at low temperature indicate the rich quantum magnetism that results from replacing spins with spin dimers on the triangular lattice.