Changes of the local distortions and colossal magnetoresistive properties of La0.7Ca0.3MnO3 induced by Ti or Ga defects

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, T-R, shifts rapidly to lower temperatures with increasing x, and the ferromagnetic transition broadens. However, the transition temperature, T-c is only slightly suppressed. Consequently T-R occurs well below T-c for x above 2%. Investigations of these materials using Mn XAFS show that changes in the local structure, parameterized by the pair-distribution width, sigma, correlate with T-c and the sample magnetization. For a given dopant, the resistivity peak occurs when sigma(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 similar to4% most of the sites are slightly distorted at low T.