Double-Exchange Magnets: Spin Dynamics In The Paramagnetic Phase

The electronic structure of perovskite manganese oxides is investigated in terms of a Kondo lattice model with ferromagnetic Hund coupling and antiferromagnetic exchange between t(2g) spins using a finite temperature diagonalization technique. Results for the dynamic structure factor are consistent with recent neutron-scattering experiments for the bilayer manganite La1.2Sr1.8Mn2O7. The susceptibility shows Curie-Weiss behavior and is used to derive a phase diagram. In the paramagnetic phase with pronounced antiferromagnetic fluctuations charge carriers form ferromagnetic polarons.