Magnetic properties of Fe-doped and Cu-doped Nb28O70

Abstract We report on AC magnetic susceptibility measurements on Fe- and Cu-doped Nb 28 O 70 , a semiconductor that contains channels of octahedral interstices which are the preferential sites for point defects segregation in the structure. The AC susceptibility displays peaks and undulations below 15 K which indicate strong correlations between the moments of localized electrons, for both kinds of dopants. We argue that localization and spin correlations are associated with the concurrent effects of electron–lattice interactions and superexchange. The effect of electron–lattice interactions on spin coupling is apparently necessary to explain the similar coupling energies observed for spins in both Cu- and Fe-doped samples.