Spin, charge, and lattice states in layered magnetoresistive oxides

Colossal magnetoresistive materials are perovskite-related mixed-valent (Mn 3+/Mn4+) manganese oxides that exhibit both spontaneous (at a Curie transit ion) and magnetic field-induced insulator-metal transitions. In concert wit h the dramatic changes in electrical conductivity, these oxides exhibit lar ge lattice anomalies, ferro- and antiferromagnetism, charge ordering of the Mn3+/Mn4+ sites, etc. In this article, we discuss how a particular class o f these manganite materials, naturally layered manganites La2-2xSr1+2xMn2O7 , has allowed us to experimentally probe many of these tightly coupled phen omena. In particular, we examine the structure-property relationships that determine the critical magnetic ground states, we discuss how conductivity and its field dependence can test prevailing models for the magnetoresistan ce effect, and we explore the interplay and competition between charge- and magnetic-order on both long- and short range length scales. Finally, we pr esent evidence from neutron and X-ray scattering that these short range cha rge correlations are essential to the mechanism of colossal magnetoresistan ce in naturally layered manganites.