Field-induced ferromagnetic order and colossal magnetoresistance in La1.2Sr1.8Mn2O7: A La139 NMR study

In order to gain insights into the origin of colossal magnetoresistance (CMR) in manganese oxides, we performed a La139 NMR study in the double-layered compound La1.2Sr1.8Mn2O7. We find that above the Curie temperature TC=126K, applying a magnetic field induces a long-range ferromagnetic order that persists up to T=330K. The critical field at which the induced magnetic moment is saturated coincides with the field at which the CMR effect reaches a maximum. Our results therefore indicate that the CMR observed above TC in this compound is due to the field-induced ferromagnetism that produces a metallic state via the double exchange interaction.