# Spin-orbit entangled j=1/2 moments in Ba$_2$CeIrO$_6$ -- a frustrated fcc quantum magnet

## TL;DR

Ba$_2$CeIrO$_6$ is an almost ideal j=1/2 Mott insulator with frustrated fcc lattice magnetism, exhibiting antiferromagnetic order driven by Kitaev interactions and magneto-elastic effects, revealing complex spin-orbit physics.

## Contribution

This work identifies Ba$_2$CeIrO$_6$ as a near-ideal model for j=1/2 moments and uncovers the role of Kitaev exchange and lattice distortions in its magnetic ordering.

## Key findings

- Less than 1% deviation from ideal j=1/2 state.
- Antiferromagnetic order at 14K despite high Curie-Weiss temperature.
- Enhanced geometric frustration due to next-nearest neighbor exchange.

## Abstract

We establish the double perovskite Ba$_2$CeIrO$_6$ as a nearly ideal model system for j=1/2 moments, with resonant inelastic x-ray scattering indicating a deviation of less than 1% from the ideally cubic j=1/2 state. The local j=1/2 moments form an fcc lattice and are found to order antiferromagnetically at $T_N$=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, indicated by ab initio theory. Magnetic order is driven by a bond-directional Kitaev exchange and by local distortions via a strong magneto-elastic effect - both effects are typically not expected for j=1/2 compounds making Ba2CeIrO6 a riveting example for the rich physics of spin-orbit entangled Mott insulators.