Geometric magnetic frustration in the R -type ferrite SrSn₂ Ga_1.3 Cr_2.7 O₁₁ and the spinel-based chromates

SrSn₂ Ga_1.3 Cr_2.7 O₁₁ is shown to be a strongly geometrically frustrated magnet through measurement of its temperature-dependent magnetic susceptibility and specific heat. This material, determined by powder neutron diffraction to have an R -type ferrite crystal structure, has S=3/2 Cr³⁺ ions in isolated Kagomé planes with no magnetic ions in the intermediary structural layers. The Kagomé planes contain 10% nonmagnetic Ga³⁺. This structure type makes the chromate family of frustrated magnets unusual because it displays four spinel-based structural variants with the same basic magnetic lattice and different effective dimensionalities and degrees of frustration. A comparison of the structures and magnetic characteristics is presented.