Quantum Spin dynamics of the Bilayer Ferromagnet La(1.2)Sr(1.8)Mn2O7
  We construct a theory of spin wave excitations in the bilayer manganite La(1.2)Sr(1.8)Mn2O7 based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. 
Nic Shannon, Tapan Chatterji, Fatiha Ouchni and Peter Thalmeier