Hyperhoneycomb Iridate β-Li2IrO3 as a Platform for Kitaev Magnetism

A complex iridium oxide beta-Li2IrO3 crystallizes in a hyperhoneycomb structure, a three-dimensional analogue of honeycomb lattice, and is found to be a spin-orbital Mott insulator with J(eff) = 1/2 moment. Ir ions are connected to the three neighboring Ir ions via Ir-O-2-Ir bonding planes, which very likely gives rise to bond-dependent ferromagnetic interactions between the J(eff) = 1/2 moments, an essential ingredient of Kitaev model with a spin liquid ground state. Dominant ferromagnetic interaction between J(eff) = 1/2 moments is indeed confirmed by the temperature dependence of magnetic susceptibility chi(T) which shows a positive Curie-Weiss temperature theta(CW) similar to +40 K. A magnetic ordering with a very small entropy change, likely associated with a noncollinear arrangement of J(eff) = 1/2 moments, is observed at T-c = 38 K. With the application of magnetic field to the ordered state, a large moment of more than 0.35 mu(B)/Ir is induced above 3 T, a substantially polarized J(eff) = 1/2 state. We argue that the close proximity to ferromagnetism and the presence of large fluctuations evidence that the ground state of hyperhoneycomb beta-Li2IrO3 is located in close proximity of a Kitaev spin liquid.