H. M. Palmer and C. Greaves
The crystallographic and magnetic structures of Fe0.5Cu0.5Cr2S4 have been examined in order to generate a possible mechanism to explain the CMR behaviour of such spinels. Neutron powder diffraction has revealed that the tetrahedrally coordinated Fe3+ and Cu+ ions are fully ordered, with the magnetic moments of Fe3+ (µ300=–1.97(5) µB) aligned antiparallel to those of the octahedrally coordinated Cr3+ (µ300=1.49(3) µB). The results suggest a new CMR mechanism, which involves exchange on three cation centres (‘triple exchange’) and requires antiferromagnetic coupling of magnetic spins, in contrast to the manganite perovskites for which double-exchange provides a ferromagnetic state.