Co doping effect on the crystal structure, magnetoresistance and magnetic properties of an (La0.7Ba0.3)(Mn1-xCox)O-3 system with x=0-1.0

The effects of chemical size and valence on the crystal structure, magnetor esistance and magnetic properties were investigated in a perovskite-like (L a0.7Ba0.3)(Mn1 - xCox)O-3 system with x = 0 - 1.0. The samples with x less than or equal to 0.3 and x less than or equal to 0.9 are of a cubic phase, bur an orthorhombic phase is observed in the samples with 0.4 less than or equal to x less than or equal to 0.8, The lattice parameter, a, based on th e cubic phase, at first decreases with increasing x up to 0,7, then slightl y varies as x greater than or equal to 0.8. The metal-insulator transition shows up in the compositions for x less than or equal to 0.2, and subsequen tly disappears as x greater than or equal to 0.3. All samples exhibit ferro magnetism at low temperatures, but the long-range magnetic ordering in pure manganate (x = 0) becomes a cluster glass-type (short-range) ferromagnetic order due to the introduction of Co. The saturation magnetization decrease s with increasing Co content for x less than or equal to 0.7, then increase s as x greater than or equal to 0.7. Our recent study using an X-ray absorp tion spectroscope has revealed that Co2+/Co3+ and Mn3+/Mn4+1 but no Co4+ or Mn2+ ions were observed in this entire series. The magnetotransport and ma gnetic properties are closely related to the average valences of Mn and Co ions. (C) 2000 Elsevier Science B.V. All rights reserved.