Spectacular doping dependence of interlayer exchange and other results on spin waves in bilayer manganites

We report the measurement of spin waves in the bilayer colossal magnetoresistive manganites La_2−2xSr_1+2xMn₂O₇ with x = 0.30, 0.35, and 0.40. For x = 0.35 and 0.40 the entire acoustic and optic dispersion relations are well described by those for a bilayer Heisenberg Hamiltonian with nearest-neighbor exchange only, which is explained together with the spin-wave lifetimes by the double exchange model. The in-plane exchange depends weakly on x, but that between the planes of a bilayer changes by a factor of 4, directly revealing a change from mixed d_3z²−r² and d_x²−y² orbital character to mostly d_x²−y².