Critical analysis of magnetically semi-disordered systems: critical exponents at various transitions

The critical analysis of various magnetically semi-disordered compounds with spinel structure is reported. The magnetic equation of state is valid in the vicinity of the different observed transitions and the critical exponents are determined by different methods. At the magnetic-paramagnetic transition, the critical exponents and are larger and smaller respectively than those found for 3D Heisenberg ferromagnets. The exponent variations are explained phenomenologically and allow a classification of the compounds to be made according to the degree of disorder. At low temperature, for some materials a mixed state occurs below the re-entrant transition, characterized by critical exponents, while for other materials local canted states exist, where the transverse spin components block progressively, revealing an absence of a true phase transition. At high temperature, a well established spin-cluster regime occurs in some cases. Two transitions with critical exponents are evident, the magnetic-cluster and the cluster-paramagnetic transitions.