Critical behaviour and magnetic interactions in the diluted systems ZnxHg1-xCr2Se4 (0 <= x <= 1)

We study the magnetic properties and the critical behaviour of the B-spinel ZnxHg1-xCr2Se4 in the concentration range 0 less than or equal to x less t han or equal to 1. Using the mean field theory and the probability distribu tion of the interactions, the exchange integrals were calculated up to seco nd nearest neighbours. The critical region is studied by the high-temperatu re series expansion extrapolated with the Fade approximants method. The cri tical temperatures the critical exponents associated with the magnetic susc eptibility (gamma) and the correlation length (nu) are obtained. The values of T-c and T-N obtained by this method are in accordance with those predic ted by magnetic measurements. The values of y and v are sensitive to the di lution ratio x. For the compounds situated in the long-range order (LRO) re gion (0 less than or equal to x less than or equal to 0.4 and 0.6 less than or equal to x less than or equal to 1), they are close to those of 3D Heis enberg model. For a concentration situated in the perturbed phase (x = 0.5) , gamma and nu deviate rapidly from the values predicted by this model and approach the values found in re-entrant systems. (C) 2001 Elsevier Science B.V. All rights reserved.