The x-ray structural, resistive, magnetic, and Mn55 NMR methods are used to investigate the ceramic magnetoresistive lanthanum manganite perovskites La0.6Sr0.2Mn1,2−xFexO3 (x=0, 0.02, 0.05, and 0.1) annealed at 1170 and 1500°C. It is found that increasing the Fe content decreases the metal-semiconductor phase transition temperature Tms and the Curie temperature TC, and increases the peak of the magnetoresistive effect near these phase transitions and results in growth of the peak at low temperatures, where the magnetoresistive effect is due to tunneling transitions of carriers between crystallites. The wide asymmetric Mn55 NMR spectrum, whose resonance frequency shifts with increasing x to lower frequencies, confirms the high-frequency electron-hole exchange between the Mn3+ and Mn4+ ions and the high degree of lattice imperfections, including not only vacancies but also clusters. The hysteresis in the field dependences of the magnetization at 4.2K is due to a change in the fraction of low-spin Mn2+ in clusters, whose magnetism appears below 42K. The increase of the activation energy with increasing Fe content is explained by the influence of these ions on the structural defect density, the charge carrier concentration, and the electron-hole exchange between heterovalent manganese ions in B positions.

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