Spin Correlations Of The Magnetoresistive Bilayer Manganite La1.2sr1.8mn2o7

We review our recent neutron and X-ray scattering studies of the 40% hole doped bilayer manganite La1.2Sr1.8Mn2O7,, which exhibits colossal magnetoresistance above the ferromagnetic transition st T-C similar to 113 K. In the paramagnetic region, static diffuse scattering reveals the existence of correlated charges. However, the good agreement of the in-plane spin correlations with the conventional model for quasi two-dimensional XY magnets indicates that the metal-insulator transition is driven by the growth of magnetic correlations. Across the transition, the competition between antiferromagnetic superexchange and ferromagnetic double exchange, observed as canted correlations between spins in neighboring layers within each bilayer, is strongly reduced, indicating a direct link of the double exchange interaction to the hole mobility.