Antiferromagnetic Short Range Order in a Two-Dimensional Manganite Exhibiting Giant Magnetoresistance

The bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$ exhibits a transition between nearly insulating paramagnetic and metallic ferromagnetic states at $T_C=126\mathrm{}\mathrm{K}$. We use magnetic neutron scattering to show that the paramagnetic state contains long-lived antiferromagnetic clusters coexisting with ferromagnetic critical fluctuations. The principal implication is that at least in two dimensions, Anderson localization effects must also be taken into account in attempts to explain giant magnetoresistance in manganites.