A study of the critical behaviour of a normal ferrimagnetic spinel by high-temperature series expansions

The critical properties of a ferrimagnetic spinel system (AB(2)X(4), A and B are magnetic ions) are studied by the method of exact high-temperature series expansions. Terms up to seventh order were computed for the magnetic susceptibility chi = Sigma(7)(n=0) a(n)(1/k(B)T)(n). The calculations are given for the three nearest neighbours' exchange integrals J(AA), J(AB) and J(BB). The Pade approximants method is used to estimate the critical exponent. associated with the magnetic susceptibility. A net variation of. with exchange couplings has been observed. This variation presents some unusual characteristics. The magnetic asymmetric interactions and the competition between the exchange interactions are important for the magnetic phase transition in ferrimagnetic spinels. We make comparisons with experiment by studying real Heisenberg spinel ferrite systems ACr(2)S(4) (A=Fe, Co). The results of gamma and T-c obtained by the present approach are in agreement with the experimental values.