The layered compound ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ $\mathrm{}(x=0.3\mathrm{})\mathrm{}$ consists of bilayers of metallic ${\mathrm{MnO}}_2$ sheets separated by insulating material. The compound exhibits markedly anisotropic magnetoresistance at temperatures well below the three-dimensional magnetic ordering temperature $T_c=90\mathrm{}\mathrm{K}$ in addition to colossal magnetoresistance around $T_c.$ 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.

• Received 31 August 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R14693