Evidence for collective spin dynamics above the ordering temperature

We have measured the temperature dependence of the electron paramagnetic resonance (EPR) of (Formula presented)(Formula presented)(Formula presented) 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 (Formula presented), 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., (Formula presented), (Formula presented), or (Formula presented) 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 (Formula presented)(Formula presented)(Formula presented) air annealed, (Formula presented)∼270 K) corresponds to correlated moments with an effective spin of ∼30 at T=280 K.