  Experimental evidence supports the double-exchange (DE) mechanism for both
in-plane and c-axis conductivity in the colossal-magnetoresistive (CMR) layered
manganite La1.4Sr1.6Mn2O7. Below TC, the data determine both the DE and
antiferromagnetic (AF) superexchange between bilayers. These agree with recent
spin-wave data, such that the ratio of intra- to inter-bilayer DE constants is
very close to the spin-independent conductance anisotropy of ~300. The
conductivity is shown to be proportional to the square of the measured
magnetization over a wide range of fields and for temperatures below the
in-plane ferromagnetic ordering temperature, TC. This dependence is shown to be
consistent with DE coupling and earlier zero-field studies of La1.2Sr1.8Mn2O7
near TC, which used neutron scattering for the local magnetization. A mixed AF
and spin-flop state (similar to the intermediate state of type-I
superconductors) is a rigorous prediction of the data and modeling.
