Crystal Structures, Magnetic and Electrical Properties of Manganese Perovskites

The Y0.67Ca0.33Mn1-yAlyO3 system with y ≤ 0.2, crystallizes in an orthorhombic type structure having Pnma space group. The La1-xPbxMnO3 perovskites with 0.24 ≤ x 0.40 have a rhombohedral structure of R 3c–type. The Y0.67Ca0.33Mn1-yAlyO3 samples show a mictomagnetic type behaviour. The analysis of charge compensation mechanism suggests that the number of oxygen vacancies increases with Al content. A semiconducting type behaviour was also shown. The contributions of double exchange mechanism to magnetic ordering in La1-xPbxMnO3 increase as the lead content is higher. A nearly ferromagnetic type ordering was shown for the sample with x=0.4. The number of Mn4+ ions fits well with the Pb2+ ones, suggesting that the change in Mn valence state is the dominant mechanism for charge compensation. A transition from metallic to semiconducting type behaviour is shown at temperatures by≅ 130 K lower than the Curie points. Magnetoresistivities up to ≅ 50 % were determined in fields of 60–70 kOe.