Percolative transition on ferromagnetic insulator manganites: Uncorrelated to correlated polaron clusters

We report an atomic-scale study on the ferromagnetic insulator manganite ${\mathrm{LaMnO}}_{3.12}$ using $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ perturbed angular correlation spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a Jahn-Teller (JT) distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition (rhombohedral-orthorhombic). Two distinct regimes of JT distortions were found: a high-temperature regime where uncorrelated polaron clusters with severe distortions of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra survive up to $T\ensuremath{\approx}800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a low-temperature regime where correlated regions have a weaker JT-distorted symmetry.