Unusual substitutional propertiesof Cu in bulk polycrystalline samples of La0.7 Ca0.3 Mn1-x Cux O3-d

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Анотація

The transport and magnetoresistive properties of bulk polycrystalline samples of the La_0.7Ca_0.3Mn_1-xCu_xO_3-d (x ≤ 0.15) system are studied in the temperature range 77 K-300 K. All the samples investigated 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 of the resistivity peak, T_p, first decreases from 193 K (x=0) to 108 K (x=0.10) and then gradually grows, reaching 120 K at x=0.15. Significant temperature broadening of the resistive transition as well as anomalous behavior of the peak value of the resistivity are observed near x=0.10. The unusual properties of the La_0.7Ca_0.3Mn_1-xCu_xO_3-d system are well explained in terms of a mixed valence of the Cu ions.