Spin dynamical properties of the layered perovskite La1.2Sr1.8Mn2O7

Inelastic neutron-scattering measurements were performed on a single crystal of the layered colossal magnetoresistance (CMR) material La1.2Sr1.8Mn2O7 (T-C similar to 120 K). We found that the spin wave dispersion is almost perfectly two-dimensional with the in-plane spin stiffness constant D similar to 151 meV Angstrom. The value is similar to that of similarly doped La1-xSrxMnO3 though its T-c is three times higher, indicating a large renormalization due to low dimensionality. There exist two branches due to a coupling between layers within a double-layer. The out-of-plane coupling is about 30% of the in-plane coupling though the Mn-O bond lengths are similar. (C) 1999 Elsevier Science Ltd. All rights reserved.