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 La-139 NMR study in the double-layered compound La1.2Sr1.8Mn2O7. We find that above the Curie temperature T-C = 126 K, applying a magnetic field induces a long-range ferromagnetic order that persists up to T = 330 K. 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 T-C in this compound is due to the field-induced ferromagnetism that produces a metallic state via the double exchange interaction.