ORDER-DISORDER AND MAGNETIC CLUSTERS IN SOME MIXED-METAL PEROVSKITES

The temperature dependence of the Fe-57 and Sn-119 Mossbauer spectra a nd the zero-field-cooled and field-cooled magnetic susceptibilities of SrLaFeSnO6, SrEuFeSnO6, BaLaFeSnO6 and CaLaFeSnO6 show the existence of unusual magnetic relaxation behaviour. In the case of CaLaFeSnO6 th ere is evidence for a non-random distribution of the Fe and Sn, althou gh the other compounds are fully disordered. It is proposed that compe tition between the first- and second-nearest-neighbour superexchange i nteractions leads to the formation of magnetic clusters which can grow with decrease in temperature until long-range antiferromagnetic order is achieved.