Exchange integrals and magnetic short range order in the system CdCr2S4(1-x) Se4x

High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice, The calculations are developed in the framework of the random phase approximation and are given for both nearest and next-nearest neighbour exchange integrals J(1) and J(2), respectively. Our results are given up to order 6 in beta = (k(B)T)(-1) and are used to study the paramagnetic region of the ferromagnetic spinel CdCr2S4(1-x)Se4x. The critical temperature T-c and the critical exponents y and v associated with the magnetic susceptibility chi (T) and the correlation length xi (T) respectively are deduced by applying the Pade approximant methods, The results as a function of the dilution x obtained by the present approach are found to be in excellent agreement with the experimental ones. (C) 2001 Elsevier Science B.V. All rights reserved.