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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-x A x MnO3 (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, T c. GMR is generally a maximum close to T c or the insulator−metal (I−M) transition temperature, T im. The T c and %MR are markedly affected by the size of the A site cation, 〈r A〉, thereby affording a useful electronic phase diagram when T c or T im is plotted against 〈r A〉. 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 〈r A〉 or the eg band width, and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field.

Effect of particle size on the electron transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated. While the ferromagnetic Tc, low field magnetic susceptibility, and insulator-metal transition are markedly affected by the particle size, the maximum magnetoresistance exhibited by the samples near Tc is not sensitive to the particle size. However, the magnetoresistance at 4.2 K increases with decrease in particle size, suggesting a substantial contribution by the grain boundaries. Preliminary measurements on La0.7Sr0.3MnO3 samples of different particle sizes also corroborate the above conclusions.

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