Abstract
The magnetoresistive properties of La0.7Ca0.3MnO3 change rapidly when Ti or Ga is substituted on the Mn site for concentrations, x, from 1 to 10%. The samples exhibit colossal magnetoresistance (CMR) and the resistivity increases dramatically with dopant concentration. The temperature of the resistivity peak, TR , shifts rapidly to lower temperatures with increasing x, and the ferromagnetic transition broadens. However, the transition temperature, Tc is only slightly suppressed. Consequently TR occurs well below Tc for x above 2%. Investigations of these materials using Mn XAFS show that changes in the local structure, parameterized by the pair-distribution width, σ, correlate with Tc and the sample magnetization. For a given dopant, the resistivity peak occurs when [sgrave]2 decreases below a critical value. Both dopants produce extended defects which increases the resistivity of the nearby material considerably. The data suggest that even at ∼4% most of the sites are slightly distorted at low T.
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