Spin glassy behaviors and isolated spin dimers in \textit{BaCr}$_{9p}$\textit{Ga}$_{12}_{-}_{9p}O_{19}$

Theoretical results suggested that quantum fluctuations may induce glassy states (spin jam states) in defect-free spin systems. Recently our experimental results indicated the existence of spin jam states in SrCr$_{9}_{p}$Ga$_{12-9}_{p}$O$_{19}$ (SCGO($p))$ in which the Cr$^{3+}$ ions form two-dimensional (2D) triangular lattice of bi-pyramids. Here we report a isostructural new system BaCr$_{9}_{p}$Ga$_{12-9}_{p}$O$_{19}$ (BCGO($p))$ with 0.4\textless $p$\textless 0.9. Neutron diffraction results show that BCGO($p)$ is isostructural as SCGO($p)$ but with a larger $c $lattice indicating that BCGO($p)$ is more 2D. BCGO($p)$ exhibits similar glassy behaviors as SCGO($p)$ but with higher freezing temperature. Very high Curie-Weiss temperature and frustration index were also observed in BCGO($p)$ suggesting that BCGO($p)$ is one of the most frustrating system. Inelastic neutron scattering results show that BCGO($p)$ has dispersionless magnetic excitations at an energy of 16.5(1) meV arising from 4$f_{vi}$-4$f_{vi}$ spin dimers. These spin dimers are adjacent spins in neighboring triangular lattice planes which separate the 2D interacting triangular lattice of bi-pyramids. These results indicate that BCGO($p)$ is a new good candidate for studying spin jam states.