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.
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