The results of inelastic and quasi-elastic neutron scattering investigations on the 40% hole-doped quasi-2D bilayer manganites La_1.2Sr_1.8Mn₂O₇ have been reviewed. The complete set of exchange interactions have been determined on the basis of a localized Heisenberg model. However, the spin wave dispersion in La_1.2Sr_1.8Mn₂O₇ shows softening close to the zone boundary and are also heavily damped especially close to the zone boundary and deviate from that expected for a simple Heisenberg model. A minimal double exchange model including quantum corrections can reproduce these effects qualitatively but falls short of quantitative agreement.

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