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Magnetic Ordering and Ergodicity in the Cu_2Te_2O_5X_2 Frustrated Spin System

We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu_2Te_2O_5X_2 family. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coupled Cu^2+ spins and (ii) coupled tetrahedra within one-dimensional chains, in both cases involving mean-field coupling to other chains. The results show that the intracluster interactions are stronger than those between clusters, but the inter-cluster interactions are strong enough to suppress the intra-cluster singlet formation, so that Cu_2Te_2O_5X_2 compounds are true 3D systems of coupled spins. We found no clear evidence on the existence of a spin gap in the excitation spectrum. Though Cu^2+ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length ξ extends beyond the single tetrahedral cluster dimension, Cu_2Te_2O_5X_2 compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagomé and pyrochlore antiferromagnets.

magnetically frustrated spin system; Cu_2Te_2O_5X_2; magnetic susceptibility; specific heat

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