Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La 0.3 Ln 0.3 Sr 0.3 Mn 1.1 O 3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, 55 Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R 3̅ c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La 3+ , Pr 3+ , Nd 3+ , Sm 3+ , or Eu 3+ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor ( T ms ) and ferromagnet–paramagnet ( T C ) phase transitions and the increase in electrical resistivity ρ and activation energy E a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3 d 4 (Mn 3+ )–2 p 6 (O 2– )–3 d 3 (Mn 4+ )– V ( a ) –3 d 4 (Mn 3+ ). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric 55 Mn NMR spectra support the high-frequency electronic double exchange Mn 3+ (3 d 4 ) ↔ O 2– (2 p 6 ) ↔ Mn 4+ (3 d 3 ) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.