Mixed-valence manganites

Mixed-valence manganese oxides (R(1-x)A(x))MnO3 (R = rare-earth cation, A = alkali or alkaline earth cation), with a structure similar to that of pero vskite CaTiO3, exhibit a rich variety of crystallographic, electronic and m agnetic phases. Historically they led to the formulation of new physical co ncepts such as double exchange and the Jahn-Teller polaron. More recent wor k on thin films has revealed new phenomena, including colossal magnetoresis tance near the Curie temperature, dense granular magnetoresistance and opti cally-induced magnetic phase transitions. This review gives an account of t he literature on mixed-valence manganites, placing new results in the conte xt of established knowledge of these materials, and other magnetic semicond uctors. Issues addressed include the nature of the electronic ground states , the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric an d magnetic polaron formation, magnetic localization, the role of cation dis order and the Jahn-Teller effect. Sample preparation, and the properties of related ferromagnetic oxides are also discussed.