Ordered stack of spin valves in a layered magnetoresistive perovskite

The layered compound La_2-2xSr₁₊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=90K 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 T. To our knowledge, 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-polarized tunneling between the magnetic layers and suggest that the material is a bulk stack of spin-valve devices.