Spin-orbit-entangled frustrated magnetism in fcc Ba₂(Yb,Nd)NbO₆ double perovskites

Original · EN

The search for candidate Kitaev materials has largely focused on 4d and 5d transition-metal compounds with various lattice geometries. In contrast, investigations of rare-earth 4f systems have thus far been restricted mainly to honeycomb and triangular lattices. In this work, we investigate the rare-earth-based double perovskites Ba₂YbNbO₆ and Ba₂NdNbO₆, which crystallize in a face-centered cubic structure. Magnetization and heat-capacity measurements establish isolated j eff = 1/2 Kramers doublet ground states arising from strong spin-orbit coupling (SOC) and crystal electric-field effects, which are further supported by density-functional theory calculations. Millikelvin-temperature thermodynamic measurements reveal long-range magnetic order with moderate frustration in both compounds. The emergence of magnetic order may be understood within an order-by-disorder scenario, as theoretically proposed for rare-earth fcc lattices with finite Kitaev interactions. Our results thus identify Ba₂YbNbO₆ and Ba₂NdNbO₆ as promising rare-earth spin-orbit-entangled magnets and motivate further experimental and theoretical investigations aimed at determining the complete exchange tensor to elucidate the microscopic origin of the underlying magnetic interactions.