Extrinsic and intrinsic features above T-c in layered manganite: La1.2Sr1.8Mn2O7

We have performed EPR and DC magnetization measurements on single crystals of La1.2Sr1.8Mn2O7. Similar to previous authors, we found an increase in the magnetization, M, near 300 K, a temperature well above T-C similar to 125 K. Previously, this high temperature( similar to 300 K) feature has been suggested to be intrinsic to the system. We have observed at these high temperatures the appearance of new ferromagnetic resonances, FMR. Within the experimental error the volume of sample required to explain the extra M and the intensity of the FMR is the same. We conclude that the increase in M and the FMR are due to the presence of extrinsic Ruddlesden-Popper (RP) phases and not to intrinsic effects as has been claimed. Furthermore, the resonance field of the EPR line due to the La1.2Sr1.8Mn2O7 phase presents a measurable temperature-dependent anisotropy up to similar to 450 K. The anisotropy is proportional to M, independent of the extrinsic RP phase content, and intrinsic to the system. We show that demagnetization effects can explain only part of the anisotropy. The role of short-range magnetic ordering in the observed intrinsic features cannot be disregarded, however. (C) 2000 Elsevier Science B.V. All rights reserved.292416711