Magnetic properties of the triangular-lattice antiferromagnets Ba3R B9 O18 (R=Yb, Er)

Frustration-induced strong quantum fluctuations, spin correlations, and interplay between competing degrees of freedom are some of the key ingredients that underlie exotic states with fractional excitations in quantum materials. Rare-earth-based two-dimensional magnetic lattices possessing a crystal electric field, spin-orbit coupling, anisotropy, and electron correlation between rare-earth moments offer a new paradigm in this context. Herein, we present crystal structure, magnetic susceptibility, and specific heat results accompanied by crystal electric field calculations on polycrystalline samples of Ba3RB9O18 (R=Yb, Er), in which R3+ ions form a perfect triangular lattice. The localized R3+ spins show neither long-range magnetic order nor spin-glass behavior down to 1.9 K in Ba3RB9O18. Magnetization data reveal pseudospin Jeff=1/2 (Yb3+) degrees of freedom in the Kramers doublet state and a weak antiferromagnetic interaction between Jeff=1/2 moments in the Yb variant. On the other hand, the effective moment μeff=8.8μB was obtained from the Curie-Weiss fit of the low-temperature susceptibility data in Ba3ErB9O18, which suggests the admixture of higher-crystal-electric-field states with the ground state. The Curie-Weiss fit of low-temperature susceptibility data for the Er system unveils the presence of a bit stronger antiferromagnetic interaction between Er3+ moments compared with its Yb3+ analog. Ba3ErB9O18 does not show long-range magnetic order down to 500 mK. Furthermore, our crystal electric field calculations based on the thermodynamic data suggest the presence of a small gap between the ground and first excited Kramers doublets. The broad maximum around 4 K in the specific heat at zero field is attributed to the thermal population of the first crystal electric field excited state in Ba3ErB9O18.