Lattice effects and phase competition in charge ordered manganites

Abstract

A complete characterization of the magnetotransport and magnetoelastic properties of two classical examples of half doped manganites, Nd0.5Sr0.5MnO3 (NSMO) and La0.5Ca0.5MnO3 (LCMO), was carried out by electrical resistivity, magnetoresistance, thermoelectric power, electron spin resonance (ESR), and thermal expansion measurements. We have identified a mixed paramagnetic (charge localized) plus ferromagnetic (FM) (conductive) state between the low temperature antiferromagnetic (AF) and high temperature paramagnetic (itinerant) phases, which in the case of LCMO exists over a temperature range of about 100 K. The magnetic field completely separates the FM and AF phases, suppressing the intermediate paramagnetic-localized phase. This mixed state is only observed in a narrow temperature interval for NSMO, and shows how the relative strength of the competing phases can be tuned by the lattice distortion. We also observed a large gap at the charge ordering temperature in the activation energy of the resistivity, which is almost independent of the sample. ESR results will be presented to show that this technique could be a very useful tool with which to investigate the multiphase microscopic state characteristic of manganites.