Detailed relationship between local structure, polarons, and magnetization for La1-xCaxMnO3 (0.21 ≤ x ≤ 0.45)

We present detailed local structure measurements (using the extended x-ray absorption fine structure technique) for the colossal magnetoresistive material La1-xCaxMnO3 (0.21 < x < 0.45) as a function of temperature and magnetic field. The local distortions of the Mn-O bonds are parameterized using sigma, the width of the Mn-O pair-distribution function (PDF). After subtracting thermal phonon contributions, we show that the contributions to sigma(2) from polaron and Jahn-Teller (JT) distortions, sigma(2)(JT/polaron), are a universal function of the magnetization, independent of how the magnetization is achieved via changes in temperature or magnetic field. However this universal behavior is only observed for B fields >= 2 T, likely as a result of domain canting in low B fields. The resulting curve is well described by two straight lines with significantly different slopes. These regimes represent two distinctly differ distortions of the oxygen octahedra about the Mn. For low magnetizations up to similar to 65% of the theoretical maximum magnetization, M-T, the slope is low and the distortion removed as the sample becomes magnetized is small-we argue this arises from polarons which have a low distortion around two (or possibly three) Mn sites. At high magnetizations large distortions per Mn site are removed as these sites become magnetized. The data are also analyzed in terms of a two Mn-O peak distribution using experimental standards for Mn-O. The results agree well with recent neutron PDF results but not with some earlier results. We discuss the limitations of assuming a two peak distribution in view of the two distortions needed to describe the Mn-O distortions as a function of T and B for B >= 2 T. It is likely that there is a distribution of longer bonds. Finally we show that with increasing B field, the Mn-Mn peak also has a small B-field-induced change-a measure at the unit cell level of magnetostriction but find that there is no observable B-field-induced change in the Mn-La/Ca pair distribution for fields up to similar to 10 T.