Spin-orbit-entangled frustrated magnetism in fcc Ba${}_2$(Yb,Nd)NbO${}_6$ double perovskites

Abstract

The search for candidate Kitaev materials has largely focused on 4$d$ and 5$d$ transition-metal compounds with various lattice geometries. In contrast, investigations of rare-earth 4$f$ systems have thus far been restricted mainly to honeycomb and triangular lattices. In this work, we investigate the rare-earth-based double perovskites Ba${}_2$YbNbO${}_6$ and Ba${}_2$NdNbO${}_6$, which crystallize in a face-centered cubic structure. Magnetization and heat-capacity measurements establish isolated $j_{\mathrm{eff}}=1/2$ 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 Ba${}_2$YbNbO${}_6$ and Ba${}_2$NdNbO${}_6$ 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.

13 pages and six figures