Geometric Magnetic Frustration in Crystal Lattices: Synthesis and Magnetic Characterization of Breathing Pyrochlore Candidates CuAlCr4S8 and CuAlCr4Se8

Abstract

The breathing pyrochlore lattice is a relatively recently discovered variant of the extensively studied pyrochlore. Materials exhibiting this structure possess the chemical formula AA’B4X8 and form a lattice of interpenetrating corner-sharing tetrahedra of alternating sizes due to the differences in atomic radii of the A and A’ atoms that occupy the neighbouring sites. The difference in tetrahedra size can be quantified by the breathing factor B_f≡d'/d . This lattice exhibits magnetic frustration as a result of its geometry, as the tetrahedral structure prevents all nearest neighbour interactions from being satisfied, and the introduction of the breathing factor breaks the symmetry of the nearest neighbour Hamiltonian, providing an arena for studying interesting magnetic interactions. Here, I present an experimental study of the breathing pyrochlore candidates CuAlCr4S8 and CuAlCr4Se8. I provide the procedure for synthesis using solid state techniques, as well as a structural analysis using X-ray diffraction and Rietveld refinement. I also perform a magnetic characterization of the samples using SQUID magnetometry, where M(H) and M(T) measurements are conducted. Through analysis of these measurements, I conclude CuAlCr4S8 forms a nearly pure breathing pyrochlore with no magnetic impurities and B_f=1.087. This is the first reporting of this material. The sample undergoes a significant transition at T_c≈26K which appears to be first order in nature and possibly antiferromagnetic. This material has a Curie-Weiss temperature θ_CW=-61.93K. Analysis of CuAlCr4Se8 shows it forms a mixed site pyrochlore structure, with a small paramagnetic impurity Cr0.68Se ≈2.5%. This material exhibits multiple characteristic temperatures and spin glass-like behaviour with θ_CW=17.14K.