| 总结: | Magnetization and magnetotransport measurements have been used to study the composition dependence of the electronic properties of the Ruddlesden-Popper phases Sr2NdMn2O7 and Sr1.9Nd1.1Mn2O7. Although their behaviour differs in detail, both compounds show a colossal magnetoresistance (CMR) effect (>10 000% in 14 T) in the temperature range 4.2 ≤ T/K ≤ 100. However, neither material shows a transition to a ferromagnetic state above 4.2 K, and both materials have higher resistivities (>103 Ω cm for 4.2 ≤ T/K ≤ 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 explained by a doubleexchange mechanism.
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