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An effect of microstructure on magnetoresistive properties of manganite ceramics (La0.65Sr0.35)0.8Mn1.2O3±Δ with superstoichiometric manganese has been studied after sintering in the temperature range of 800–1500 °C. Increase in sintering temperature to 1400 °C, an enormous growth of grains, densification of material, and rise in magnetoresistance peak from 1% to 4.8% are observed. At the same time, sintering at 1500 °C leads to a sharp increase in the grain size more than 100 μm, release of excess manganese on grain boundaries, and almost two times magnetoresistance peak rise. We propose that such radical changes are due to the presence of superstoichiometric manganese which plays a crucial role in the formation of ceramics microstructure and properties.

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Samples of La0.7Mn1.3O3±Δ and (La0.65Sr0.35)0.8Mn1.2O3±Δ with particle sizes ranging from 6to200nm are obtained using cold isostatic pressing. The coercive force of the experimental samples is determined from the field dependences of the resistance and dynamic magnetic susceptibility. It is determined by two methods that the contribution of the surface layer to the magnetic characteristics of manganites is composition dependent. It is shown experimentally for the first time that the coercive force in manganites reaches its highest values with particle size of the order of 70nm for both compositions and vanishes completely for lanthanum manganite with ∼6nm particles as a result of reaching a superparamagnetic state.