Effect of Nonstoichiometry on Properties of La1−tMnO3+δ: III. Magnetic Order Studied by Powder Neutron Diffraction

Abstract The magnetic order, moments, and ordering temperatures for La1−tMnO3+δdepend strongly on its nonstoichiometry. Powder neutron diffraction samples were obtained via precursor based synthesis, annealing in atmospheres with controlled oxygen partial pressures, and adequate quenching or slow cooling. The samples are representative for most of the nonstoichiometric domain. Three modifications of La1−tMnO3+δwere studied. First, the reduced phase with Jahn–Teller deformed MnIIIis orthorhombic (ORT1) and orders antiferromagnetically (Axmode), typicallyTN≈ 140 K and μAF≈ 3.4 μB(0.00 ≤t≤ 0.08). All samples of this type exhibit parasitic ferromagnetism. Second, the fully oxidized samples have average Mn oxidation state around 3.2, and have rhombohedral (RH) crystal structure with ferromagnetic order at low temperatures;TC≈ 213 ± 10 K and μF= 3.9 ± 0.1 μBfor La0.96MnO3.05. Third, for intermediate oxygen contents an orthorhombic structure (ORT2) occurs, with minor distortions of the MnO6-octahedra. For LaMnO3.08of ORT2-type, ferromagnetic order (Fymode) occurs belowTC= 125 ± 10 K. The ordered magnetic moment, μF= 2.46 ± 0.04 μBis rather small. LaMnO3.15also takes the ORT2-type structure at low temperature, as proved by weak superstructure reflections. At 200 ± 20 K, LaMnO3.15undergoes a first order transition to the RH-type. The ordered magnetic moment, if present, is very small and in any case far from expectations for MnIII/MnIVmixtures. It is proposed that the approximately 3:1 ratio between MnIIIand MnIVatoms in LaMnO3.15gives rise to a frustrated system with largely spin glass behaviour