EPR evidence for premonitory charge-ordering fluctuations in hydrothermally grown Pr_0.57Ca_0.41Ba_0.02MnO₃ nanowires

Abstract

Electron paramagnetic resonance (EPR) and magnetic properties of nanowires of Pr_0.57Ca_0.41Ba_0.02MnO₃ (PCBMO) are studied and compared with those of the bulk material. PCBMO nanowires with diameter of 80-90 nm and length of ~3.5 µm were synthesized by a low reaction temperature hydrothermal method and the bulk sample was prepared following a solid-state reaction route. The samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The bulk PCBMO manganite exhibits charge order at 230 K along with a ferromagnetic transition at 110 K. However, superconducting quantum interference device measurements on the PCBMO nanowires show a complete 'melting'of charge ordering and a ferromagnetic transition at 115 K. 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 nanowires at the temperature corresponding to the charge-ordering transition, i.e., 230 K. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanowires even though the static charge order is absent, thus heralding the occurrence of charge order in the bulk sample.