Ba8 MnNb6 O24: A model two-dimensional spin- 5/2 triangular lattice antiferromagnet

We successfully synthesized and characterized the triangular lattice antiferromagnet Ba8MnNb6O24, which comprises equilateral spin-52Mn2+ triangular layers separated by six nonmagnetic Nb5+ layers. The detailed susceptibility, specific heat, elastic and inelastic neutron scattering measurements, and spin-wave theory simulation on this system reveal that it has a 120 ordering ground state below TN=1.45K with in-plane nearest-neighbor exchange interaction ≈0.11meV. While the large separation 18.9 Å between magnetic layers makes the interlayer exchange interaction virtually zero, our results suggest that a weak easy-plane anisotropy is the driving force for the km=(1/3,1/3,0) magnetic ordering. The magnetic properties of Ba8MnNb6O24, along with its classical excitation spectra, contrast with the related triple perovskite Ba3MnNb2O9, which shows easy-axis anisotropy, and the isostructural compound Ba8CoNb6O24, in which the effective spin-12Co2+ spins do not order down to 60 mK and in which the spin dynamics shows signs of strong quantum effects.