Abstract
It is established by x-ray, resistive, NMR, and magnetoresistive studies of ceramic and thin-film laser perovskites La0.7Mn1.3O3 with “excess” manganese that the real structure of these compounds contains heterovalent manganese ions and cationic and anionic vacancies and clusters, the magnetism and resistivity of the latter being manifested near 42 K. The broad, asymmetric 55Mn and 139La NMR spectra of the ceramics attest to the presence of high-frequency electron–hole exchange between the heterovalent manganese ions and to a high degree of imperfection and mesoscopic inhomogeneity of nonstoichiometric lanthanum manganite perovskites. The differences of the metal–semiconductor phase transition temperatures and activation energy of the ceramics and films is explained by the different oxygen nonstoichiometry, structural imperfection, and, accordingly, densities of charge carriers and excitons. The low-field magnetoresistive effect of the ceramic is explained by tunneling at grain boundaries. It is conjectured that the decrease in resistance in magnetic field is due to an increase in charge carrier density due to weakening of the electron–hole interaction in excitons. An anomaly of the resistance and magnetoresistive effect observed near 42 K is explained by the presence of clusters.
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