Magnetic dilution in the strongly frustrated kagome antiferromagnet ${\mathrm{SrGa}}_{12\mathrm{-}\mathit{x}}$${\mathrm{Cr}}_{\mathit{x}}$${\mathrm{O}}_{19}$

A study of the structural and magnetic characteristics of the strongly frustrated kagome system ${\mathrm{SrGa}}_{12\mathrm{-}\mathit{x}}$ ${\mathrm{Cr}}_{\mathit{x}}$ ${\mathrm{O}}_{19}$ (1<×<9) is reported with focus on the effects of the magnetic ion dilution. High-resolution powder neutron diffraction shows no evidence of long-range magnetic ordering down to 1.5 K and a nearly random substitution of Cr by Ga atoms within the octahedral sites. An intrinsic quasibidimensional structure integrated by kagome blocks is inferred. ac and dc magnetic susceptibility measurements down to 1.6 K make evident the existence of a freezing phenomenon similar to that of spin glasses with the freezing temperature ${\mathit{T}}_{\mathit{f}}$, decreasing as the degree of dilution of the ${\mathrm{Cr}}^{3+}$ ions increases. The extrapolated value of ${\mathit{T}}_{\mathit{f}}$ for the fully occupied lattice is 4.15 K, indicating that the nondiluted oxide could be considered as a prototype of geometrically frustrated magnetic systems. The magnetic frustration parameter Θ/${\mathit{T}}_{\mathit{f}}$≊135±5 is independent of the degree of dilution, giving further support to the idea of an intrinsic structural origin of the frustration in this system. Both magnetic susceptibility measurements and ESR studies show a Curie-Weiss-like behavior in the high-temperature range with magnetic correlations starting around 100 K.