Spin dynamics of quasi-2D ferromagnet La1.2Sr1.8Mn2O7

Neutron scattering investigations of the magnetic ordering and spin waves h ave been carried out on the bilayer manganite La1.2Sr1.8Mn2O7 which shows c olossal magnetoresistance (CMR) effect close to the ferromagnetic phase tra nsition T-C approximate to 128 K. The dispersions of both the acoustic and the optic modes along [100] and [001] have been measured and by using a eff ective localised spin model the complete set of exchange parameters has bee n determined. The strong anisotropy in the exchange interactions suggests t hat La1.2Sr1.8Mn2O7 can be considered as a quasi two-dimensional ferromagne t consisting of weakly coupled ferromagnetic bilayers. Although an effectiv e localised spin model can explain the spin wave dispersion for small momen tum and energy transfers close to the zone centre, it is no longer valid fo r larger momentum and energy transfers close to the zone boundary. The ener gy widths of the spin waves are found to be much larger than the instrument resolution showing the dual localised-itinerant character of the electroni c system. (C) 2001 Elsevier Science BY Ail rights reserved.