Magnetism on ideal triangular lattices in NaBaYb(B O3)2

We report the anisotropic magnetic properties of single crystals of the triangular lattice magnet NaBaYb(BO3)2. In this material, a layered [Yb(BO3)2]∞ framework is sandwiched by Na+ or Ba2+ layers in alternation. The space group is centrosymmetric, R3̄m, with no detectable disorder and Yb3+-based easy axis Kramers doublets forming layered triangular lattice planes. A specific-heat anomaly indicates a second-order phase transition at T=0.41(2)K that is suppressed to less than 0.15 K in a 0.1-T magnetic field. With a change in entropy of only 0.6(2)% of Rln2 at this transition, NaBaYb(BO3)2, however, retains 94(1)% of Rln2 entropy to the lowest temperature accessed (T=0.15K). Strong frustration and the potential for quantum magnetism are implied.