Transport and Magnetic Properties of the Perovskites La_1-yMnO₃ and LaMn_1-zO₃

Single-phase samples of La 1 - y MnO 3 - (3 y /2)+ δ with 0 ≤ y ≤ 0.09 and LaMn 1 - z O 3 - (3 z /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 GdFeO 3 ( Pbnm ) for small values of δ and the rhombohedral structure of LaAlO 3 ( R 3̄ c ) for larger values of δ. Below a temperature T c, 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 T c . At higher values of δ, a sharp increase of T c to a value greater than the Weiss temperature θ p for the paramagnetic phase indicates a first-order transition on lowering T through T c from a polaronic to an itinerant-electron conductor within the matrix between trapped-hole clusters; lack of full ferromagnetic alignment of the Mn spins is found despite the higher value of T c . Transport properties show an increasing trapping out of mobile polaronic holes on cooling to T c with a release of these holes below T c . Samples with z > 0 trap out mobile holes more strongly and remain polaronic at all temperatures except below T c at highest values of δ in the matrix between hole-rich clusters.