Low-energy magnetic excitations in double-layered manganite

We have investigated the magnetic correlations and low-energy magnetic excitations in a layered manganite $\rm La_{1.2}Sr_{1.8}Mn_{2}O_{7}$ which shows colossal magnetoresistance (CMR) behaviour. Our results confirm the coexistence of both ferromagnetic and antiferromagnetic correlations above $T_{\rm C} \approx 126\;{\rm K}$. In addition we have discovered dispersive low-energy magnetic excitations of energy of 0.1–0.4 meV propagating along [001] in the ferromagnetic phase below TC which we have analyzed within an effective exchange model of weakly coupled ferromagnetic double layers