Evolution of spin-wave excitations in ferromagnetic metallic manganites

Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic A(1-x)A(x)(')MnO(3) manganites (where A and A(') are rare- and alkaline-earth-metal ions), which when combined with previous work elucidate the systematics of the interactions as a function of carrier concentration x, on-site disorder, and strength of the lattice distortion. The long-wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange (J(4)/J(1)) that controls the zone boundary magnon softening changes systematically with x, but does not depend on the other parameters. None of the prevailing models can account for these behaviors.