Ba8CoNb6 O24: A spin- 12 triangular-lattice Heisenberg antiferromagnet in the two-dimensional limit

The perovskite Ba8CoNb6O24 comprises equilateral effective spin-12 Co2+ triangular layers separated by six nonmagnetic layers. Susceptibility, specific heat, and neutron scattering measurements combined with high-temperature series expansions and spin-wave calculations confirm that Ba8CoNb6O24 is basically a two-dimensional magnet with no detectable spin anisotropy and no long-range magnetic ordering down to 0.06 K. In other words, Ba8CoNb6O24 is very close to be a realization of the paradigmatic spin-12 triangular Heisenberg model, which is not expected to exhibit symmetry breaking at finite temperatures according to the Mermin and Wagner theorem.