Antiferromagnetic fluctuations and short-range order in a Kagomé lattice

We report neutron-scattering measurements on ${\mathrm{SrCr}}_{8\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{4+\mathit{x}}$${\mathrm{O}}_{19}$, a layered compound containing Kagomé planes of S=3/2 ${\mathrm{Cr}}^{3+}$ ions. From magnetic-susceptibility data, the Curie-Weiss constant is large and negative; ${\mathrm{\theta }}_{\mathrm{CW}}$≃-500 K. Even so, our measurements for a sample with x=0.87 reveal only short-range antiferromagnetic order with a correlation length ξ=7±2 Å≊2a, where a is the inter-Cr spacing. Below 8 K, there is a spin-freezing transition to a state where the magnetic fluctuations have large amplitude and a spectrum (averaged over reciprocal space) characteristic of two-dimensional antiferromagnets with ξ≫a.