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 th e 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 s uggested to be intrinsic to the system. We have observed at these high temp eratures the appearance of new ferromagnetic resonances, FMR. Within the ex perimental error the volume of sample required to explain the extra M and t he 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 resonan ce field of the EPR line due to the La1.2Sr1.8Mn2O7 phase presents a measur able temperature-dependent anisotropy up to similar to 450 K. The anisotrop y is proportional to M, independent of the extrinsic RP phase content, and intrinsic to the system. We show that demagnetization effects can explain o nly part of the anisotropy. The role of short-range magnetic ordering in th e observed intrinsic features cannot be disregarded, however. (C) 2000 Else vier Science B.V. All rights reserved.