Spin-orbit entangled j= 12 moments in Ba2CeIrO6: A frustrated fcc quantum magnet

We establish the double perovskite Ba2CeIrO6 as a nearly ideal model system for j=1/2 moments, with resonant inelastic x-ray scattering indicating that the ideal j=1/2 state contributes by more than 99% to the ground-state wave function. The local j=1/2 moments form an fcc lattice and are found to order antiferromagnetically at TN=14K, more than an order of magnitude below the Curie-Weiss temperature. Model calculations show that the geometric frustration of the fcc Heisenberg antiferromagnet is further enhanced by a next-nearest neighbor exchange, and a significant size of the latter is indicated by ab initio theory. Our theoretical analysis shows that magnetic order is driven by a bond-directional Kitaev exchange and by local distortions via a strong magnetoelastic effect. Both, the suppression of frustration by Kitaev exchange and the strong magnetoelastic effect are typically not expected for j=1/2 compounds making Ba2CeIrO6 a riveting example for the rich physics of spin-orbit entangled Mott insulators.