CRYSTAL AND MAGNETIC-STRUCTURES OF THE COLOSSAL MAGNETORESISTANCE MANGANATES SR2-XND1+XMN2O7 (X=0.0, 0.1)

Neutron powder diffraction techniques have been used to refine the cry stal and magnetic structures of the Ruddlesden Popper (RP) compounds S r2-xNd1-xMn2O7 (x=0.0, 0.1), which show colossal magnetoresistance bel ow 100 K. Samples of both compositions have been shown to be biphasic, although in each case both phases are of the RP type and have very si milar structural parameters. Only one phase in each sample shows long range magnetic order. For x=0.0, the Mn cations and the Nd cations in the rocksalt layers of the structure are antiferromagnetically ordered in a noncollinear magnetic structure below 28 K; the Nd cations withi n the perovskite blocks do not show long range magnetic order. In the temperature range 28 less than or equal to T/K less than or equal to 1 37, only the Mn cations are ordered. The decoupling of the Nd cations is accompanied by a rotation of the Mn moments from a direction close (phi=27 degrees) to the z axis into the xy plane. A similar temperatur e dependence is seen in the case x=0.1, although the ordered component s of the cation moments are much reduced and the low temperature phase has a collinear magnetic structure with the moments aligned along z. The magnetotransport properties of these samples are discussed in the light of their crystal and magnetic structures.