Electron transport studies in rhombohedral series of Al-doped LaMnO3+δ: an effective medium approach
| Autor: | A Banerjee, R V Krishnan |
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| Rok vydání: | 2000 |
| Předmět: | |
| Zdroj: | Journal of Physics: Condensed Matter. 12:7887-7903 |
| ISSN: | 1361-648X 0953-8984 |
| DOI: | 10.1088/0953-8984/12/36/304 |
| Popis: | We report the electron transport studies in the rhombohedral LaMn1-xAlxO3+δ series (0≤x≤20%) of samples. Though all the samples are ferromagnetic below a transition temperature (TC), only the samples with x≤5% exhibit a metal-insulator transition (MIT) at TC while the samples with x≥10% are all semiconducting above and below TC. The sample with x = 7.5% is a borderline case where MIT at TC is immediately taken over by semiconducting behaviour at lower temperatures. Thus a progressive crossover from ferromagnetic-metallic state to ferromagnetic-insulating state is observed 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 from a double exchange dominated regime to an exclusively superexchange regime, preserving the rhombohedral symmetry, makes the present series an important and unique one for the study of electron transport in the colossal magnetoresistance manganites. An effective medium approach is employed to explain the resistivity behaviour in this series over the whole temperature range, which gives strong support for polaronic conduction in all the samples. This polaronic conduction justifies the presence of dynamic Jahn-Teller effects in the samples and the change in the character of polarons across TC illustrates the role of electron-lattice interaction as well as its coupling to magnetic states (core spins) of the samples. The dominance of double exchange is evident from metallic resistivity in the samples with x≤5% which have Mn4+ much above the percolation threshold for metallicity. However the observed non-trivial temperature dependence of metallic resistivity could not be explained by double exchange alone and it is shown in this study that one needs to take into account superexchange interactions even in the double exchange dominated regime to understand the electron transport, thus supporting the current understanding in these pervoskite manganites. |
| Databáze: | OpenAIRE |
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