Evidence for collective spin dynamics above the ordering temperature in ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3+\mathrm{\delta }}$

We have measured the temperature dependence of the electron paramagnetic resonance (EPR) of ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{MnO}}_{3+\mathrm{\delta }}$ for 0.0≤x≤1 at 9.2 and 35 GHz from 270 to 800 K. We vary δ by oxygen and argon annealing. We observe that the intensity of the EPR line decreases exponentially for T above the ordering temperature ${\mathit{T}}_{\mathit{c}}$, with an activation energy which varies with x and δ. We find that the EPR data below ∼600 K cannot be attributed to an individual Mn ion of any valency, i.e., ${\mathrm{Mn}}^{2+}$, ${\mathrm{Mn}}^{3+}$, or ${\mathrm{Mn}}^{4+}$ for all annealing conditions. The theoretical models of these ‘‘colossal’’ magnetoresistance compounds has not yet focused on the ground-state spin properties, and an interpretation of these data is therefore still lacking. We suggest that the explanation of these EPR data may provide useful tests of the correct model of the magnetization and magnetotransport properties in these compounds. The maximum EPR intensity per formula unit (for ${\mathrm{La}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_3$ air annealed, ${\mathit{T}}_{\mathit{c}}$∼270 K) corresponds to correlated moments with an effective spin of ∼30 at T=280 K. © 1996 The American Physical Society.