Abstract
We have used inelastic neutron scattering to measure the spin wave dispersion througout the Brillouin zone of the double exchange ferromagnet La
0.7Pb
0.3MnO
3. The dispersion is well described by that for the double exchange Hamiltonian. The Curie temperature calculated for a local moment ferromagnet with the observed dispersion agrees with experiment within 15%. In the paramagnetic phase of the two-dimensional analogue La
1.2Sr
1.8Mn
2O
7, we have observed long-lived antiferromagnetic cluster coexisting with ferromagnetic fluctuations. This effect must be included in theories that attempt to explain giant magnetoresistance in manganites, at least in two dimensions.