COLOSSAL MAGNETORESISTANCE IN SR2-XND1+XMN2O7 (X=0.0,0.1)

Magnetization and magnetotransport measurements have been used to stud y the composition dependence of the electronic properties of the Ruddl esden-Popper phases Sr2NdMn2O7 and Sr1.9Nd1.1Mn2O7. Although their beh aviour differs in detail, both compounds show a colossal magnetoresist ance (CMR) effect (>10 000% in 14 T) in the temperature range 4.2 less than or equal to T/K less than or equal to 100. However, neither mate rial shows a transition to a ferromagnetic state above 4.2 K, and both materials have higher resistivities (> 10(3) Omega cm for 4.2 less th an or equal to T/K less than or equal to 100) than the metallic oxides previously found to show CMR. In view of the low conductivity and the absence of ferromagnetism, the CMR of these phases is not readily exp lained by a double-exchange mechanism.