Evidence of dynamic Jahn-Teller effects in ferromagnetism of rhombohedral Al-substituted lanthanum manganite

Abstract

We report the structural, magnetic and electron transport properties of a new series of compounds where we have substituted Al3+ in the Mn site of rhombohedral, cation deficient LaMnO3+. All the samples crystallize in rhombohedral (Rc) symmetry with minimal changes in lattice parameters. Mn4+ content gradually decreases from ~25% with increase in Al content and samples with x15% are stoichiometric without any detectable Mn4+. A gradual transition from the ferromagnetic-metallic state to ferromagnetic-insulating state is observed with increase in Al substitution. This indicates a progressive crossover from the double exchange dominated regime indicated by metallicity to the superexchange regime indicated by the absence of Mn4+ across the series. Ferromagnetism in the samples with no Mn4+ is because of ferromagnetic Mn3+-O-Mn3+ interactions which is possible in the presence of dynamic Jahn-Teller effects in Rc symmetry. A semiquantitative estimate of TC was carried out on the basis of near-neighbour exchange interactions and a good agreement with the observed TC is reported. Through this the importance of dynamic Jahn-Teller effects coupled with Mn3+-O-Mn3+ interactions giving rise to ferromagnetism is shown for the whole series. Samples with x7.5% show a second transition in ac susceptibility at lower temperatures and from the preliminary measurements their origin is attributed to the dynamics of small ferromagnetic clusters arising due to random Al substitution in some of the infinite ferromagnetic matrix.