MAGNETIC DYNAMICS IN COLOSSAL MAGNETORESISTIVE PEROVSKITE MANGANITES

We have measured the spin-wave dispersion throughout the Brillouin zon e of the double-exchange ferromagnet La0.7Pb0.3MnO3 by using inelastic neutron scattering. An unexpectedly simple Heisenberg Hamiltonian wit h solely a nearest-neighbour coupling accounts for the entire dispersi on relation. The value of the Curie temperature T-C calculated from th e derived coupling constant agrees with the experimental value to with in 15%. The results, including damping of the zone boundary spin waves near T-C, can be fully understood in the framework of the double-exch ange Hamiltonian. In the paramagnetic phase of the two-dimensional ana logue La1.2Sr1.8Mn2O7, we have observed long-lived antiferromagnetic c lusters coexisting with ferromagnetic fluctuations. This effect must b e included in theories that attempt to explain giant magnetoresistance in manganites, at least in two dimensions.