EPR Evidence for Premonitory Charge-Ordering Fluctuations in Nanomanganite Pr_0.57Ca_0.41Ba_0.02MnO₃

Abstract.

Electron paramagnetic resonance (EPR) and magnetic properties of nanocrystalline Pr_0.57Ca_0.41Ba_0.02MnO₃ are studied and compared with those of the bulk material. The nanoscale samples prepared by the sol–gel method were annealed at different temperatures to obtain particles of different sizes. The crystallite and particle sizes were estimated by X-ray diffraction and transmission electron microscopy and found to be approximately equal to 30, 60 and 100 nm. Magnetization studies on the 60 and 100 nm particles show charge-ordering at 230 K similar to the bulk sample prepared by solid-state reaction, while in the 30 nm particles the charge-ordering signature is absent. At low temperatures all the three nanosamples show ferromagnetic transitions. This result is confirmed by the EPR intensity behavior as well. However, the EPR line width, which is reflective of the spin dynamics, shows a shallow minimum for the 30 nm particles at the temperature of 230 K, corresponding to the charge-ordering transition. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanoparticles of 30 nm size even though the static charge order is absent, thus heralding the occurrence of charge order in the larger particles.