Changes of the local distortions and colossal magnetoresistive
properties of La(0.7)Ca(0.3)MnO(3) induced by Ti or Ga defects
The magnetoresistive properties of La0.7Ca0.3MnO3 change rapidly when Ti or Ga are substituted on the Mn site for concentrations, x, from 1 to 10 percent. 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 percent. Investigations of these materials using Mn XAFS show that changes in the local structure, parametrized by the pair-distribution width, sigma, correlate well with Tc 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 materials considerably. The data suggest that even at x~;4 percent, most of the sites are slightly distorted at low T.