Novel insights into the magnetic behavior of non-stoichiometric LaMnO3+δ nanoparticles

LaMnO3+delta is one of the classical-formal nonstoichiometric systems in materials science that, for delta = 0, shows antiferromagnetic ordering by super-exchange interactions between Mn3+ ions. Nevertheless, the chemical reactivity of solids goes through different pathways when the particle size is reduced to the nanometric range, leading to the modification of the physico-chemical properties of materials. It is demonstrated here that octagonal LaMnO3+delta nanoparticles of average size 20 nm significantly modify their non-stoichiometry, microstructure and magnetotransport properties and are thus able to tune their behavior, as a function of the cationic vacancy concentration. For delta = 0, LaMnO3 nanoparticles, even with only Mn3+, become ferromagnetic, whereas for delta = 0.23, i.e. La0.93Mn0.93O3, due to a cationic diffusion mechanism, weaker ferromagnetic interactions appear promoting the emergence of magnetoresistance. This accommodation of compositional variations triggers Mn3+ -> Mn4+ oxidation and facilitates, as observed by atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy, the displacement of some La atoms around their normal site that can introduce restrictions in double exchange Mn3+-O2--Mn4+ interactions.