Spin, Charge, and Lattice States in Layered Magnetoresistive Oxides

Colossal magnetoresistive materials are perovskite-related mixed-valent (Mn 3+ /Mn 4+ ) manganese oxides that exhibit both spontaneous (at a Curie transition) and magnetic field-induced insulator−metal transitions. In concert with the dramatic changes in electrical conductivity, these oxides exhibit large lattice anomalies, ferro- and antiferromagnetism, charge ordering of the Mn 3+ /Mn 4+ sites, etc. In this article, we discuss how a particular class of these manganite materials, naturally layered manganites La 2 - 2 x Sr 1+2 x Mn 2 O 7, has allowed us to experimentally probe many of these tightly coupled phenomena. 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 magnetoresistance effect, and we explore the interplay and competition between charge- and magnetic-order on both long- and short range length scales. Finally, we present evidence from neutron and X-ray scattering that these short range charge correlations are essential to the mechanism of colossal magnetoresistance in naturally layered manganites.