Microstructure evolution and magnetoresistance of the A-site ordered Ba-doped manganites

The microstructure, crystal structure, and magnetotransport properties of microsized and nanosized Badoped manganites have been investigated. A “two-step” reduction-reoxidation procedure has been used to obtain nanosized ceramic manganite Nd0.70Ba0.30MnO3 (II). The parent microsized manganite Nd0.70Ba0.30MnO3 (I) was prepared by usual ceramic technology in air. Then the sample was annealed in vacuum. The grain size of the reduced sample, determined by scanning electron microscopy, decreased from ∼5 μm down to ∼100 nm. To obtain the oxygen stoichiometry nanosized sample, the Nd0.70Ba0.30MnO2.60 was again annealed in air. It is established that the (I) sample is a pseudocubic perovskite, whereas (II) is tetrahedral as a consequence of Nd3+ and Ba2+ ions as well as the ordering of oxygen vacancies. The (I) sample is a ferromagnet with T C ≈ 140 K. It has metal-insulator transition at T MI ≈ 135 K and a peak of magnetoresistance ∼50% in a field of 9 kOe. For the (II) sample, the critical points of phase transitions move to higher temperatures, T C ≈ 320 K and T MI ≈ 310 K. The magnetoresistance of the (II) sample at room temperature (T ≈ 293 K) is about 7% in a field of 9 kOe. The magnetotransport properties are interpreted in the framework of the nanosized effect.