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We have studied the magnetic dilution and electronic nature of Zn doping on the Mn site in the colossal magnetoresistant material La0.7Pb0.3MnO3 (x≤0.3). Small non-magnetic Zn2+ doping tends to separate the system into ferromagnetic clusters to weaken the long-range ferromagnetic order and to reduce the Curie temperature. The spin polarizability of the x=0–0.3 samples is estimated to be 0.97–1.00, indicating that the x=0–0.3 samples are the spin polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x≥0.1) induces the metal–insulator transition and destroys the metallic state with long-range ferromagnetic order. [Copyright &y& Elsevier]

Abstract: We measured transport properties in layered cobalt oxides , that are parent materials of the superconductor , in order to reveal so-far unsettled origin of these electronic and magnetic features. Various sodium nonstoichiometric samples () have been carefully synthesized to control the carrier density and the electronic state of cobalt ions. We found a large negative magnetoresistance (MR) effect in the vicinity of and 1.0. The and 1.0 samples show negative MR, and , respectively, at 90kOe and 4K. These negative MR suggest the pseudogap formation due to the charge ordering in layers, such as and , all of which exhibit similar behaviors. The suppression of the pseudogap by a magnetic field causes the large negative magnetoresistance. [Copyright &y& Elsevier]