Giant Magnetoresistance and Related Properties of Rare-Earth Manganates and Other Oxide Systems

Giant magnetoresistance (GMR), which was until recently confined to magnetic layered and granular materials, as well as doped magnetic semiconductors, occurs in manganate perovskites of the general formula Ln1-xAxMnO3 (Ln = rare earth; A = divalent ion). These manganates are ferromagnetic at or above a certain value of x (or Mn4+ content) and become metallic at temperatures below the curie temperature, Tc. GMR is generally a maximum close to Tc or the insulator−metal (I−M) transition temperature, Tim. The Tc and %MR are markedly affected by the size of the A site cation, 〈rA〉, thereby affording a useful electronic phase diagram when Tc or Tim is plotted against 〈rA〉. We discuss GMR and related properties of manganates in polycrystalline, thin-film, and single-crystal forms and point out certain commonalities and correlations. We also examine some unusual features in the electron-transport properties of manganates, in particular charge-ordering effects. Charge ordering is crucially dependent on 〈rA〉 or the eg band width, and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field.