Variable-range hopping conductivity and absence of a true metal–insulator transition in La0.7−δ Ca0.3Mn1−yFeyO3

Abstract

Resistivity of La0.7−δ Ca0.3Mn1−yFeyO3 (LCMFO) with y=0–0.05, δ=0 and 0.017 is investigated between 4.2–350 K, showing that La vacancies increase the effect of Fe doping by influencing electronic states through additional microscope disorder. The Shklovskii–Efros variable-range hopping conductivity, governed by generation of a soft parabolic gap in the spectrum of localized states due to Coulomb interaction between charge carriers, is observed both above and well below the ferromagnetic-to-paramagnetic transition temperature. Our results give evidence for absence of a true metal–insulator transition both in undoped material and in LCMFO.