Relationship between macroscopic physical properties and local distortions of low-doping La1-xCaxMnO3:: An EXAFS study

A temperature-dependent extended x-ray-absorption fine structure investigation of La1-xCaxMnO3 is presented for the concentration range that spans the ferromagnetic-insulator (FMI) to ferromagnetic-metal (FMM) transition region, x=0.16, 0.18, 0.20, and 0.22; the titrated hole concentrations are slightly higher, y=0.2, 0.22, 0.24, and 0.25, respectively. For this range of Ca concentrations the samples are insulating for x=0.16-0.2 and show a metal/insulator (M/I) transition for x=0.22. All samples are ferromagnetic although the saturation magnetization for the 16% Ca sample is only similar to 70% of the expected value at 0.4 T. This raises a question as to the nature of the ferromagnetic coupling mechanism in such insulating samples. We find that the FMI samples have similar correlations between changes in the local Mn-O distortions and the magnetization as observed previously for the colossal magnetoresistance (CMR) samples (0.2 <= x <= 0.5)-except that the FMI samples never become fully magnetized. The data show that there are at least two distinct types of distortions. The initial distortions removed as the insulating sample becomes magnetized are small and provide direct evidence that roughly 50% of the Mn sites (associated with the hole charge carriers) have a small average distortion/site and are magnetized first. The large Mn-O distortions that remain at low T are attributed to a small fraction (<30%) of fully Jahn-Teller-distorted Mn sites that are either unmagnetized or antiferromagnetically ordered. Thus the insulating samples are very similar to the behavior of the CMR samples up to the point at which the M/I transition occurs for the CMR materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of the sample is magnetic, implies that there must be preferred magnetized Mn sites (that involve holes) and that such sites do not percolate at these concentrations.