Exchange integrals and magnetic short range order in the system CdCr2−xGaxSe4(0⩽x⩽0.06)

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 J1 and J2, respectively. Our results are given up to order 6 in β=(kBT)-1and are used to study the paramagnetic region of the ferromagnetic spinel CdCr2−xGaxSe4. The critical temperature Tc and the critical exponents γ and ν associated with the magnetic susceptibility χ(T) and the correlation length ξ(T), respectively are deduced by applying the Padé approximate methods. The results as a function of the dilution x obtained by the present approach are found to be in agreement with the experimental ones.