Abstract
The electron paramagnetic resonance (EPR) signal in the mixed valence perovskite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3+y}$ was investigated in the paramagnetic regime for ${}^{16}$O and ${}^{18}$O isotope substituted compounds. The characteristic differences observed in EPR intensity and linewidth for the two isotope samples can be explained by a model in which a bottlenecked spin relaxation takes place from the exchange-coupled constituent ${\mathrm{Mn}}^{4+}$ ions via the ${\mathrm{Mn}}^{3+}$ Jahn-Teller ions to the lattice. For $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.2$ the ferromagnetic exchange energy $J$ exhibits a ${}^{16}\mathrm{O}{/}^{18}\mathrm{O}$ oxygen isotope effect of $\ensuremath{\sim}\ensuremath{-}10%$. The observed isotope effects suggest the presence of Jahn-Teller polarons in these materials.
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