Phase diagrams of two dilute insulating systems with competing interactions: CdCr_2xIn_2-2xS₄ and ZnCr_2xAl_2-2xS₄

Spin glass behaviour has been observed in a very large concentration range in both CdCr_2xIn_2-2xS₄ and ZnCr_2xAl_2-24 spinel systems. It is a result of the presence in these systems of competing interactions, ferromagnetic between first-nearest neighbours and antiferromagnetic between higher-order neighbours. The ratios between the interactions are such that the non-dilute x=1 compounds are ferromagnetic for the first system and helimagnetic for the second. The entire phase diagrams of both solutions have been determined by very low field AC and DC susceptibility measurements. The results for the ferromagnetic system are compared with theoretical predictions derived from a bond-diluted Ising model using the replica method and a second-order variational approximation.