Anisotropic Local Distortion of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ Through the Ferromagnetic Transition Temperature

In previous temperature-dependent EXAFS studies of the quasi-cubic manganite, La$_{1-x}$Ca$_{x}$MnO$_{3}$, excess broadening of the Mn-O pair-distribution function has been observed near and above the Curie temperature, T$_{c}$=120, attributed to the appearance of Jahn-Teller polarons. A similar, yet anisotropic, broadening is expected in the bilayer system, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$. We report a temperature-dependent, polarized EXAFS study at the Mn-K edge for the x=0.4 sample. This analysis shows similar broadening for Mn-O pairs in both the ab-plane and the c-axis, but the magnitude of the distortion in the ab-plane is about 4 times greater than along the c-axis. The data also show a small excess of broadening below T$_{c }$ as well, indicating that polaron distortions are present even at 100K where the magnetization becomes constant. Analysis of further Mn neighbors is currently underway. Details of the analysis and a discussion of the implications for CMR in the bilayer manganites will be presented. Support: NSF DMR 0301971.