Magnetoresistance of the metallic perovskite oxide LaNiO3-delta

We report a study of the magnetoresistance (MR) of the metallic perovskite oxide LaNiO3-delta as a function of the oxygen stoichiometry delta (delta l ess than or equal to 0.14), magnetic field (H less than or equal to 6 T) an d temperature (1.5 K less than or equal to T less than or equal to 25 K). W e find a strong dependence of the nature of the MR on the oxygen stoichiome try. The MR at low temperatures changes from positive to negative as the sa mple becomes more oxygen deficient (i.e. delta increases). Some of the samp les, which are more resistive, show resistivity minima at T-min approximate to 20 K. We find that in these samples the MR is positive for T > T-min an d negative for T < T-min. We conclude that in the absence of strong magneti c interaction, the negative MR in these oxides can arise from weak-localiza tion effects.