Unusual substitutional properties of Cu in bulk polycrystalline samples ofLa0.7Ca0.3Mn1-xCuxO3-delta

The transport and magnetoresistive properties of bulk polycrystalline sampl es of the La0.7Ca0.3Mn1-xCuxO3-delta (x less than or equal to0.15) system a re studied in the temperature range 77 K-300 K. All the samples investigate d exhibit the giant magnetoresistance effect associated with the transition from the ferromagnetic metallic to the paramagnetic insulating state. As a function of copper concentration, the temperature T-p of the resistivity p eak first decreases from 193 K (x=0) to 108 K (x=0.10) and then gradually g rows, reaching 120 K at x=0.15. Significant temperature broadening of the r esistive transition as well as anomalous behavior of the peak value of the resistivity are observed near x=0.10. The unusual properties of the La0.7Ca 0.3Mn1-xCuxO3-delta system are well explained in terms of a mixed valence o f the Cu ions. (C) 2001 American Institute of Physics.