Semiconductor-metal transition and magnetoresistance in La(1+x)/3Ba(2-x)/3Cu1-xMnxO3

Samples of La((1+x))(/3)Ba((2-x))Cu1-xMnxO3 (0.7 less than or equal to x le ss than or equal to 1) were fabricated by solid-state reaction. X-ray diffr action reveals a structural distortion from the cubic perovskite to orthorh ombic symmetry with increasing Cu doping content. The measurement of resist ance vs temperature dependence in zero field shows a semiconductor-metal tr ansition at T-p and a resistance minimum at low temperatures for the Cu dop ing range less than or equal to 20%. Applying a magnetic field of 5 T large ly enhances the conductivity in the whole range of T < T, and the largest m agnetoresistance effect appears around T-p. The double-exchange model is us ed to discuss the present observations in which the effects of A-site radiu s (rA) and the A-site size mismatch sigma(2) as well as the proportion of M n4+ ions are considered. We also point out that the Cu ions enter into the Mn sites in the form of Cu3+ hut not Cu2+ in order to explain the observed transition in samples with higher Cu doping contents.