SPIN-WAVES AND OTHER MAGNETIC FLUCTUATIONS IN GIANT MAGNETORESISTIVE PEROVSKITE MANGANITES

We have used inelastic neutron scattering to measure the spin wave dis persion throughout the Brillouin zone of the double exchange ferromagn et La0.7Pb0.3MnO3. The dispersion is well described by that for the do uble exchange Hamiltonian. The Curie temperature calculated for a loca l moment ferromagnet with the observed dispersion agrees with experime nt within 15%. In the paramagnetic phase of the two-dimensional analog ue La1.2Sr1.8Mn2O7, we have observed long-lived antiferromagnetic clus ters coexisting with ferromagnetic fluctuations. This effect must be i ncluded in theories that attempt to explain giant magnetoresistance in manganites, at least in two dimensions.