We search for general patterns that explain the low field magnetoresistance at low temperatures in the system A(2-x)A'xFeMoO6. The observed linear dependence of the low field magnetoresistance with the saturation magnetization for the series is related to the antisite disorder at the Fe and Mo sites. This is explained in terms of a spin dependent crossing of intragranular barriers originated from the presence of antiferromagnetic SrFeO3 patches that naturally develop when antisite disorder occurs in the double perovskite. The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.

BiMn 2 O 5 has been prepared in polycrystalline form by a citrate technique, and studied by neutron powder diffraction ͑NPD͒, specific-heat, and magnetization measurements. High-resolution NPD data show that BiMn 2 O 5 is orthorhombic ͑space group Pbam, Zϭ4) and contains infinite chains of Mn 4ϩ O 6 octahedra sharing edges, linked together by Mn 3ϩ O 5 pyramids and BiO 8 units. These units are strongly distorted with respect to those observed in other members of the RMn 2 O 5 family (R denotes rare earths͒, due to the presence of the electronic lone pair on Bi 3ϩ. BiMn 2 O 5 becomes magnetically ordered below T N ϭ39 K. The magnetic structure, investigated from low-temperature NPD data, is defined by the propagation vector kϭ(1/2,0,1/2). A group theory analysis has been carried out to determine the possible solutions for the magnetic structure. For the Mn 3ϩ ions located at the 4h site, the magnetic moments are ordered according to the basis vectors (F x ,C y ,0); for the Mn 4ϩ ions placed at the 4 f site, the basis vectors are (G x Ј ,A y Ј,0). At Tϭ1.6 K, the magnetic moments for Mn 3ϩ and Mn 4ϩ cations are 3.23(6) B and 2.51(7) B , respectively. The commensurate character of the magnetic structure of BiMn 2 O 5 , in contrast with the incommensurate structure observed for other compounds of the RMn 2 O 5 family, for which kϭ(1/2,0,k z), is discussed in terms of competing antiferromagnetic interactions, depending on the observed variation of crucial Mn-O-Mn distances.

The pressure dependence of the critical temperature T c and upper critical field H c2 (T ) has been measured up to 19 GPa in the layered superconducting material 2H-NbSe 2 . T c (P ) has a maximum at 10.5 GPa, well above the pressure for the suppression of the CDW order. Using an effective two band model to fit H c2 (T ), we obtain the pressure dependence of the anisotropy in the electron phonon coupling and Fermi velocities, which reveals the peculiar interplay between CDW order, Fermi surface complexity and superconductivity in this system.

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