Structural effects in magnetoresistive manganites and mechanism of metal-insulator transition

Colossal magnetoresistivity (CMR) is caused by a magnetic field-induced ins ulator-to-metal transition. It is arguably one of the most dramatic phenome na that occur in solids due to competing forces in a complex system. In the CMR oxides the electron-phonon coupling and spin correlations that favor c harge localization are competing against the electron kinetic energy and la ttice elasticity that prefer charge delocalization. The CMR phenomenon occu rs at the crossover point of charge-localized (insulating) and charge-deloc alized (metallic) states, where the system is particularly susceptible to e xternal stimuli such as a magnetic field. The competitions among these forc es are usually considered globally as volume averages. However, since the s ystem is complex and the phenomenon is non-linear, local fluctuations domin ate the behavior of the system near the critical point. Therefore the syste m would appear spatially inhomogeneous if the measurement is made with cert ain time and length scales. To characterize and understand such a system it is required to deploy local approaches in which the competing interactions are evaluated locally. Experimentally, the pulsed neutron pair-density fun ction (PDF) analysis is one of the methods of local structural study. Using the PDF technique it is suggested that the phase transition occurs via loc al and percolative processes. Also through the analysis of the local struct ure it is shown that the ionic size effect on the CMR phenomenon is not cau sed by a change in the band width as is usually assumed, but is due to loca l structural changes that affect polaron stability. The critical ionic size determined by this approach is in excellent agreement with experimental ob servations. The PDF results also indicate that the local structure of layer ed manganites is close to that of perovskite, suggesting that they share co mmon elements of the CMR mechanisms.