Delocalized and localized states of eg electrons in half-doped manganites

Abstract

We have studied the magnetic behaviour of half-doped manganite Y0.5Ca0.5MnO3 in an extended range of temperatures by means of magnetic susceptibility and electron spin resonance (ESR) experiments. At high temperature the system crystallizes in an orthorhombic structure. The resistivity value ~0:05 Ohm.cm at 500 K, indicates a metallic behaviour, while the Curie-Weiss dependence of the susceptivility and the thermal evolution of the ESR parameters are very well described by a model that considers a system conformed by localized Mn4+ cores,t2g^3, and itinerant, eg, electrons. The strong coupling between t2g and eg electrons results in an enhanced Curie constant and an FM Curie-Weiss temperature that overcomes the AFM interactions between the t2g^3 cores. A transition to a more distorted phase is observed at T ~500 K and signatures of localization of the eg electrons appear in the susceptibility behaviour below 300 K. A new Curie-Weiss regime is observed, where the Curie-constant value is consistent with dimer formation. Based on mean-field calculations, the dimer formation predicted as a function of the interaction strength between the t2g and eg electrons.