Magnetic properties of the triangular-lattice antiferromagnets Ba3RB9O18 (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 mu eff = 8.8 mu B 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.

Automated summary

Frustration-induced strong quantum fluctuations, spin correlations, and interplay between competing degrees of freedom are important factors in the formation of exotic states with fractional excitations in quantum materials. In this study, it was found that the rare-earth-based two-dimensional magnetic lattice Ba3RB9O18 possesses a perfect triangular lattice structure with localized R3+ spins showing pseudospin Jeff = 1/2 degrees of freedom and weak antiferromagnetic interactions. Crystal electric field calculations further suggest the presence of higher-crystal-electric-field states and a small gap between the ground and excited Kramers doublets.