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

We report the structural, magnetic and electron transport properties of a n ew series of compounds where we have substituted Al3+ in the Mn site of rho mbohedral, cation deficient LaMnO3+delta All the samples crystallize in rho mbohedral (R (3) over bar c) symmetry with minimal changes in lattice param eters. Mn4+ content gradually decreases from similar to 25% with increase i n Al content and samples with x greater than or equal to 15% are stoichiome tric without any detectable Mn4+. A gradual transition from the ferromagnet ic-metallic state to ferromagnetic-insulating state is observed with increa se in Al substitution. This indicates a progressive crossover from the doub le exchange dominated regime indicated by metallicity to the superexchange regime indicated by the absence of Mn4+ across the series. Ferromagnetism i n the samples with no Mn4+ is because of ferromagnetic Mn3+-O-Mn3+ interact ions which is possible in the presence of dynamic Jahn-Teller effects in R (3) over bar c symmetry. A semiquantitative estimate of T-C was carried out on the basis of near-neighbour exchange interactions and a good agreement with the observed T-C is reported. Through this the importance of dynamic J ahn-Teller effects coupled with Mn3+-O-Mn3+ interactions giving rise to fer romagnetism is shown for the whole series. Samples with x greater than or e qual to 7.5% show a second transition in ac susceptibility at lower tempera tures and from the preliminary measurements their origin is attributed to t he dynamics of small ferromagnetic clusters arising due to random Al substi tution in some of the infinite ferromagnetic matrix.