Spin dynamics of the quasi-two-dimensional ferromagnetic bilayer manganite La1.2Sr1.8Mn2O7 -: art. no. 014439

yWe have investigated the spin dynamics of the quasi-two-dimensional bilayer manganite La1.2Sr1.8Mn2O7, which shows colossal magnetoresistance (CMR) behavior, by inelastic neutron scattering. We have taken advantage of the recently developed multiplexing technique with a three-axis spectrometer to map simultaneously a large region of the (Q,omega) space in a low-dimensional magnetic system. We have measured the spin wave dispersion at T=16 K along [100], followed the temperature evolution of the spin wave dispersion close to T-C approximate to 128 K and also measured the scattered neutron intensities in the (Q,omega) space above T-C up to T=200 K. The magnetic inelastic intensity above T-C looks qualitatively very similar to those below T-C and have a structure in the (Q,omega) space suggesting persistent spin waves. However, although there exist well defined peaks at finite energies in constant energy slices up to the highest temperature investigated, no well defined peaks at finite energy are observed above T-C in constant-Q slices of the spin response measured in the (Q,omega) space.