Low-energy magnetic excitations in double-layered manganite La1.2Sr1.8Mn2O7

We have investigated the magnetic correlations and low-energy magnetic exci tations in a layered manganite La1.2Sr1.8Mn2O7 which shows colossal magneto resistance (CMR) behaviour. Our results confirm the coexistence of both fer romagnetic and antiferromagnetic correlations above T-C approximate to 126 K. In addition we have discovered dispersive low-energy magnetic excitation s of energy of 0.1-0.4 meV propagating along [001] in the ferromagnetic pha se below Te which we have analyzed within an effective exchange model of we akly coupled ferromagnetic double layers.