Local Probe Study of S=1 Spin Liquid Candidate Ba$_3$NiSb$_2$O$_9$

The family of hexagonal perovskites, Ba$_3M$Sb$_2$O$_9$, has attracted a considerable amount of attention in recent years, with the discovery of several spin liquid candidates. For $M=$Cu, the material is fairly disordered and likely exhibits a honeycomb-like lattice whereas in other cases the structure consists of triangular planes of spins. Three different structural phases of Ba$_3$NiSb$_2$O$_9$ have been discovered, depending on synthesis pressure [1]. Two of these phases (6HA and 6HB) consist of triangular planes of $S=1$ moments, and differ primarily by the stacking of these planes. Here, we present muon spin rotation ($\mu$SR) and $^{121}$Sb nuclear magnetic resonance (NMR) results on a high-pressure synthesis of this material, 6HB-Ba$_3$NiSb$_2$O$_9$. Most importantly, we demonstrate that there are no signs of magnetic ordering or spin freezing down to temperatures as low as 20 mK, making this material a plausible spin liquid candidate. Furthermore our NMR results are indicative of gapless excitations, consistent with previous specific heat and magnetic susceptibility results [1].\\[4pt] [1] J. G. Cheng \emph{et al.}, PRL {\bf 107}, 197204 (2011).