EPR investigation of nanosized La0.67Ca0.33MnO3−δ manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La0.67Ca0.33MnO3−δ manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn3+ and Mn4+ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.