  The layered compound La2-2xSr1+2xMn2O7 (x=0.3) consists of bilayers of
metallic MnO2 sheets separated by insulating material. The compound exhibits
markedly anisotropic magnetoresistance at temperatures well below the
three-dimensional magnetic ordering temperature TC=90 K in addition to colossal
magnetoresistance around TC. We present neutron diffraction data which show
that the magnetic structure of this material switches from antiferromagnetic
stacking of the (ferromagnetically ordered) sheets in zero field to
ferromagnetic stacking in a field of 1.5 Tesla. The data are the first to be
collected on any manganite as a function of applied field, exactly as the
magnetoresistance data themselves are collected. They provide a natural
explanation of the low-field magnetoresistance in the ordered phase in terms of
spin-polarised tunnelling between the magnetic layers and suggest that the
material is a bulk stack of spin-valve devices.
