Strong frustration and dilution-enhanced order in a quasi-2D spin glass

The interactions in the magnetoplumbite isomorph ${\mathrm{SrCr}}_8$ ${\mathrm{Ga}}_4$ ${\mathrm{O}}_{19}$ are highly frustrated. Thermodynamic data are presented and show the existence of a transition from paramagnetic to spin-glass ordering but only at ${\mathit{T}}_{\mathit{c}}$=3.3 K, despite the high Curie-Weiss temperature of 515 K. Other unusual properties include a low-temperature specific heat varying as ${\mathit{T}}^2$ and an inverse relationship of ${\mathit{T}}_{\mathit{c}}$ to magnetic-ion concentration for a set of differently prepared crystals. This result is conjectured to arise from competition between 2D frustration and a tendency toward 3D order, controlled by preferential site substitution.