Spin waves in the bilayer manganite La2-2xSr1+2xMn2O7, x=0.35

We report the measurements of low temperature spin waves in the bilayer col ossal magnetoresistive manganite La2-2xSr1+2xMn2O7 with hole doping x = 0.3 5. 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(parallel to) = 9.3 +/- 0.1 meV and 2SJ (perpendicular to) = 5.7 +/- 0.2 meV within and between planes respectively . The ratio J(perpendicular to)/J(parallel to) = 0.61 is larger than the co rresponding value of similar to 0.3 for x approximate to 0.4. This is expla ined by greater d(3,2-r2) orbital occupancy as the hole doping is reduced. The acoustic spin wave damping is proportional to the spin wave energy in t he long wavelength limit. The damping is much larger than expected from mag non-magnon or magnon-electron scattering. (C) 2001 Elsevier Science B.V. Al l rights reserved.