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

Neutron scattering investigations of the magnetic ordering and spin waves have been carried out on the bilayer manganite La1.2Sr1.8Mn2O7 which shows colossal magnetoresistance (CMR) effect close to the ferromagnetic phase transition 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 effective localised spin model the complete set of exchange parameters has been determined. The strong anisotropy in the exchange interactions suggests that La1.2Sr1.8Mn2O7 can be considered as a quasi two-dimensional ferromagnet consisting of weakly coupled ferromagnetic bilayers. Although an effective localised spin model can explain the spin wave dispersion for small momentum and energy transfers close to the zone centre, it is no longer valid for larger momentum and energy transfers close to the zone boundary. The energy widths of the spin waves are found to be much larger than the instrument resolution showing the dual localised-itinerant character of the electronic system. (C) 2001 Elsevier Science BY Ail rights reserved.