Ba NMR studies of the triangular lattice antiferromagnets Ba$_3$MSb$_2$O$_9$ (M=Co, Ni)

Ba$_3$MSb$_2$O$_9$, with M=Co, Ni are triangular lattice magnetic systems with near-neighbor antiferromagnetic exchange. For M=Co (S=1/2), the ground state is ordered and there are field-induced changes to the symmetry, whereas for the Ni (S=1) system there is no evidence for a phase transition to a lower-symmetry phase. Here we report on Ba nuclear magnetic resonance (NMR) spectroscopy and spin-lattice relaxation measurements for both systems. For example, the temperature dependence of the relaxation rate is independent of temperature for the Ni-based compound, and is similar to what is observed for the high-symmetry phase of the Co compound. The spin structures for the ordered phases of the Co material are also explored.