Investigations on the charge transport in LaMnO3+delta at low temperatures

Samples of LaMnO3+delta with 0.08 less than or equal to delta less than or equal to 0.16 have been synthesized in order to study the effect of non-stoichiometry, delta, which is related to the concentration of Mn-IV ions, on the physical properties of the material. For delta less than or equal to 0.10 the compound crystallizes in the orthorhombic perovskite-type structure, while for delta > 0.10 a rhombohedral cell is formed. All LaMnO3+delta samples with delta less than or equal to 0.12 are paramagnetic, or, below T-C, ferromagnetic semiconductors, while samples with larger values of delta exhibit a discontinuity in the temperature dependence of the resistivity, indicating a transition to a ferromagnetic metallic-like state at low temperatures. Upon further lowering of the temperature the charge carriers become localized again. These results are consistent with thermopower measurements. All samples are ferromagnetic at low temperatures. The magnetic properties can be interpreted on the basis of Goodenough's superexchange model.