The relationship between local structure and magnetization in colossal magnetoresistant manganites

The Calcium doped Lanthanum manganites (La1-xCaxMnO 3) are known to exhibit colossal magnetoresistance (CMR) in the range 0.2 < x < 0.5. Samples within this doping range also have a metal-insulator transition (TMI) at nearly the same temperature as the ferromagnetic transition temperature (Tc). In addition, these materials have a Jahn-Teller-like polaron distortion in the paramagnetic insulating phase which disappears at lower temperatures when the sample becomes magnetic.; In this study, the relationship between the local lattice distortions and magnetization was investigated using Extended X-ray Absorption Fine Structure (EXAFS) analysis at the Mn K-edge as a function of temperature (5 to 300K), applied magnetic field (0 to 11 T), and Ca concentration, x, (0.21 < x < 0.45). It was found that the decrease in local Mn-O distortion with increasing magnetization does not depend on whether the magnetization was induced by a decrease in temperature or an increase in the external magnetic field. In addition there is a marked discontinuity in the linear slope of the distortion as a function of magnetization, which is independent of calcium concentration (occurring at a normalized magnetization of 65 +/- 3% for all samples). Above this threshold, the amount of distortion removed with further incremental increases in magnetization is approximately two to three times larger than below the threshold. This has led to a proposed model in which the magnetization initially develops via small clusters of ∼2--4 locally magnetized Mn sites, with a reduced distortion and an intermediate Mn valence. These clusters, which must contain at least one hole, will be constrained by the location of the Ca ions. Thus, the aggregation of these clusters into ferromagnetic domains will lead to filamentary magnetic chains consisting of unit cells containing Ca ions. These results will be discussed in the context of earlier unexplained results.; This study presents first comprehensive investigation of the relationship between the local structure and magnetization for the CMR manganites. Furthermore, the change in the Mn-Mn bond distribution as a function of temperature and magnetic field provides a unique observation of magnetostriction at the unit cell level.