Spin glassy behaviors and isolated spin dimers in BaCr9 pGa12 - 9 pO19

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 SrCr₉ p Ga12_-9 p O₁₉ (SCGO(p)) in which the Cr3⁺ ions form two-dimensional (2D) triangular lattice of bi-pyramids. Here we report a isostructural new system BaCr₉ p Ga12_-9 p O₁₉ (BCGO(p)) with 0.4< p<0.9. Neutron diffraction results show that BCGO(p) is isostructural as SCGO(p) but with a larger clattice 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 4f_vi-4f_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.