Spin waves in the bilayer manganite la_2-2xSr _{1 + 2x}Mn₂O₇, x = 0.35

We report the measurements of low temperature spin waves in the bilayer colossal magnetoresistive manganite La_2-2xSr_{1 + 2x}Mn ₂O₇ with hole doping x = 0.35. The dispersion relations for acoustic and optic branches are reasonably well described by a Heisenberg Hamiltonian with nearest neighbour exchange only. The coupling constants are 2SJ_|| = 9.3 ± 0.1 meV and 2S⊥ = 5.7 ± 0.2meV within and between planes respectively. The ratio J⊥/J_|| = 0.61 is larger than the corresponding value of ~ 0.3 for x = 0.4. This is explained by greater d_3z2-r2 orbital occupancy as the hole doping is reduced. The acoustic spin wave damping is proportional to the spin wave energy in the long wavelength limit. The damping is much larger than expected from magnon-magnon or magnon-electron scattering.