Juxtaposition of spin freezing and long range order in a series of geometrically frustrated antiferromagnetic gadolinium garnets

Specific heat measurements in zero magnetic field are presented on a homologous series of geometrically frustrated antiferromagnetic Heisenberg garnet systems. Measurements of Gd₃Ga₅O₁₂, grown with isotopically pure Gd, agree well with previous results on samples with naturally abundant Gd, showing no ordering features. In contrast, samples of Gd₃Te₂Li₃O₁₂ and Gd₃Al₅O₁₂ are found to exhibit clear ordering transitions at 243 and 175 mK, respectively. The effects of low level disorder are studied through dilution of Gd³⁺ with nonmagnetic Y³⁺ in Gd₃Te₂Li₃O₁₂. A thorough structural characterization, using x-ray diffraction, is performed on all of the samples studied. We discuss possible explanations for such diverse behavior in very similar systems.