Glassy behavior and isolated spin dimers in a new frustrated magnet BaCr_9pGa_12-9pO₁₉

Using bulk susceptibility and neutron scattering techniques, we have studied a new frustrated magnet, BaCr_9pGa_12-9pO₁₉ [BCGO(p)], with 0.4 ≲ p ≲ 0.9. This system is isostructural to SrCr_9pGa_12-9pO₁₉ [SCGO(p)], in which the magnetic Cr³⁺ (3d³, s = 3/2) ions form a quasi-two-dimensional triangular lattice of bi-pyramids or kagome-triangle-kagome trilayers. Our bulk susceptibility data exhibit glassy behavior at temperatures much lower than the absolute values of the Curie-Weiss temperature Θ_CW ≈ -695(1) K for BCGO(p = 0.902(8)). The frustration index |Θ_CW|/T_f is as high as 190 for BCGO(p = 0.902(8)) indicating strong frustration. Our inelastic neutron scattering data on BCGO(p = 0.902(8)) reveal a dispersionless magnetic excitation centered at ℏω = 16.5(1) meV, due to a singlet to triplet excitation of spin s = 3/2 dimers. The spin dimers are formed by Cr³⁺ ions in two 4f_vi layers that lie between 12k-2a-12k (kagome-triangle-kagome) trilayers. These results indicate that BCGO(p) is another good candidate system for a strongly frustrated quasi-two-dimensional magnet.