MAGNETIC-STRUCTURES OF LAMNO3+DELTA PEROVSKITES (DELTA=0.11, 0.15, 0.26)

Oxygen excess LaMnO3+delta perovskites with a wide range of delta valu es (and Mn-IV contents) have been obtained by thermal decomposition of metal citrates followed by annealings either in air or under high oxy gen pressure (up to 200 bar). Three representative samples, with delta values of 0.11, 0.15 and 0.26 (Samples 1-3) have been studied by neut ron diffraction in the 2-250 K temperature range. The magnetic structu res, ordered magnetic moments and long-range ordering temperatures hav e been determined. Sample 1 is orthorhombic and exhibits a purely ferr omagnetic structure below 165(10) K; Samples 2 and 3 are rhombohedral and show a canted ferromagnetic structure, below T-c = 140(10) K and 6 5(10) K, respectively. The observed decrease of T-c and the magnitude of the ordered magnetic moments as delta increases has been interprete d as a consequence of the presence in the crystal structure of Mn vaca ncies, which perturb the connecting paths for the transport of holes a cross Mn-O-Mn, progressively preventing the double-exchange interactio n. Also the dramatic reduction of the observed magnetic moments for in creasing Mn-IV contents can be explained in a system which in fact sho ws spin-glass features. (C) 1997 Elsevier Science Ltd.