Electron transport studies in rhombohedral series of Al-doped LaMnO3+δ: aneffective medium approach

Abstract

We report the electron transport studies in the rhombohedral LaMn1-xAlxO3+δ series (0⩽x⩽20%) of samples. Though all the samples areferromagnetic below a transition temperature (TC), only the samples with x⩽5% exhibit ametal-insulator transition (MIT) at TC while the samples with x⩾10% are allsemiconducting above and below TC. The sample with x = 7.5% is a borderline case whereMIT at TC is immediately taken over by semiconducting behaviour at lower temperatures. Thusa progressive crossover from ferromagnetic-metallic state to ferromagnetic-insulating state isobserved in this series within the same structure. This is accompanied by a decrease in Mn4+content across the magnetic and conduction percolation thresholds. This systematic crossover froma double exchange dominated regime to an exclusively superexchange regime, preserving therhombohedral symmetry, makes the present series an important and unique one for the study ofelectron transport in the colossal magnetoresistance manganites. An effective medium approach isemployed to explain the resistivity behaviour in this series over the whole temperature range, whichgives strong support for polaronic conduction in all the samples. This polaronic conduction justifiesthe presence of dynamic Jahn-Teller effects in the samples and the change in the character ofpolarons across TC illustrates the role of electron-lattice interaction as well as its coupling tomagnetic states (core spins) of the samples. The dominance of double exchange is evident frommetallic resistivity in the samples with x⩽5% which have Mn4+ much above thepercolation threshold for metallicity. However the observed non-trivial temperature dependence ofmetallic resistivity could not be explained by double exchange alone and it is shown in this studythat one needs to take into account superexchange interactions even in the double exchangedominated regime to understand the electron transport, thus supporting the current understanding inthese pervoskite manganites.