Abstract
Single-phase samples of La1-yMnO3-(3y/2)+δ with 0 ≤ y ≤ 0.09 and LaMn1-zO3-(3z/2)+δ with 0 ≤ z ≤ 0.09 have been prepared via the citrate method with subsequent heating at different temperatures and partial pressures of oxygen. Oxygen parameters 0 ≤ δ ≤ 0.18 were obtained. At room temperature, all compositions y and z have the orthorhombic structure of GdFeO3 (Pbnm) for small values of δ and the rhombohedral structure of LaAlO3 (R3c) for larger values of δ. Below a temperature Tc, all samples show a weak ferromagnetism at δ = 0 that increases with δ, samples with y = 0 approaching a full ferromagnetic alignment of all Mn moments in the range 0.10 < δ < 0.13. At smaller δ, the orthorhombic samples contain randomly distributed superparamagnetic clusters that couple via an antiferromagnetic matrix to form a spin glass below Tc. At higher values of δ, a sharp increase of Tc to a value greater than the Weiss temperature θp for the paramagnetic phase indicates a first-order transition on lowering T through Tc f...
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