Electron Paramagnetic Resonance and Magnetization Insights into Size-Induced Charge Order ‘Melting’ in Nanoparticles of Sm0.42Ca0.58MnO3

When cooled, some manganites of the general form RE_1-xA_xMn³⁺_1-xMn⁴⁺_xO₃ where RE is a trivalent rare earth or Bi³⁺ ion and A is a divalent alkaline earth ion, undergo a ‘charge ordering’ (CO) transition resulting in a periodic ‘ordered’ arrangement of the Mn³⁺ and Mn⁴⁺ ions in the crystal lattice leading to an increase in the resistivity and antiferromagnetic ordering. Size reduction to nanoscale is an intrinsic way of destabilizing CO for achieving properties, such as room temperature colossal magnetoresistance, important for applications. Here, we address the still unsettled question in this context of whether this size induced ‘melting’ of CO in manganites is complete or if some charge order persists at short range, through comprehensive electron paramagnetic resonance (EPR) and magnetic studies of nanoparticles of Sm_0.42Ca_0.58MnO₃ and those of the bulk counterpart for comparison. The nanosized samples were prepared by microwave assisted reverse micelle method by optimizing the choice of the surfactant as well as the water to surfactant ratio to obtain particles of minimum size and polydispersity. The crystallite and particle sizes were estimated by X-ray diffraction and transmission electron microscopy. The valence states of the manganese ions on the surface of the particles were determined by X-ray photoemission spectroscopy. The near-total absence of the tell-tale signatures of CO viz. the peaks in magnetization and EPR intensity and the minimum in the EPR linewidth at the CO temperature T_CO point towards a complete disappearance of charge order in sufficiently small and monodisperse nanomanganites.