Local distortions in La0.7Ca0.3Mn1ÀbAbO3 „AÄTi and Ga... colossal magnetoresistance samples: Correlations with magnetization and evidence for cluster formation

X-ray absorption fine structure ~XAFS! measurements as a function of temperature have been carried out at the Mn K edge for La0.7Ca0.3Mn12xTixO3 and La0.7Ca0.3Mn12yGayO3 (x and y50.01 to 0.10! and correlated with transport and magnetization measurements. Most samples exhibit colossal magnetoresistance ~CMR! at low temperature. The magnetization data show a concentration dependence: the ferromagnetic phase transition broadens as x or y increases, and both the transition temperature Tc and the saturated magnetization decrease with increasing dopant concentration. The transport measurements show that the resistivity increases as x or y increases, and that the resistivity peak, which we associated with the metal-to-insulator ~MI! transition temperature TMI , moves rapidly to lower temperatures with x or y. In contrast to the La12aCaaMnO3 materials, for which the resistivity peak normally occurs very close to Tc , TMI is usually far below Tc for these cosubstituted materials. The increase in resistivity well below Tc strongly suggests the formation of clusters. We also find that TMI has a much smaller magnetic field dependence than that for La12aCaaMnO3 CMR materials. The XAFS data show that a non-Debye broadening, associated with polaron formation, also develops as T approaches Tc , as observed for other CMR samples, but the magnitude of this extra broadening, Ds , decreases with x or y, with a larger effect for Ti than for Ga. We find that for a given type of dopant, the resistivity peak occurs when s is decreased to a specific value that is essentially independent of concentration and that this corresponds to nearly the same value of the sample magnetization. These results indicate that the addition of either Ga or Ti distorts the local Mn-O environment which likely promotes cluster formation. Measurements of the absorption edge shift as a function of x for these materials do not quite follow the calculated edge positions based on concentrations, possibly suggesting small variations in O stoichiometry.