Field-Induced Ferromagnetic Order and Colossal Magnetoresistance in La_{1.2}Sr_{1.8}Mn_2O_7: a ^{139}La NMR study

摘要

In order to gain insights into the origin of colossal magneto-resistance(CMR) in manganese oxides, we performed a ^{139}La NMR study in thedouble-layered compound La_{1.2}Sr_{1.8}Mn_2O_7. We find that above the Curietemperature T_C=126 K, applying a magnetic field induces a long-rangeferromagnetic order that persists up to T=330 K. The critical field at whichthe induced magnetic moment is saturated coincides with the field at which theCMR effect reaches to a maximum. Our results therefore indicate that the CMRobserved above T_C in this compound is due to the field-induced ferromagnetismthat produces a metallic state via the double exchange interaction.