GIANT MAGNETORESISTANCE AND RELATED PROPERTIES OF RARE-EARTH MANGANATES AND OTHER OXIDE SYSTEMS

Giant magnetoresistance (GMR), which was until recently confined to ma gnetic layered and granular materials, as well as doped magnetic semic onductors, occurs in manganate perovskites of the general formula Ln(1 -x)A(x)MnO(3) (Ln = rare earth; A = divalent ion). These manganates ar e 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) tran sition 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 electro nic phase diagram when T-c or T-im is plotted against [r(A)]. We discu ss GMR and related properties of manganates in polycrystalline, thin-f ilm, and single-crystal forms and point out certain commonalities and correlations. We also examine some unusual features in the electron-tr ansport properties of manganates, in particular charge-ordering effect s. Charge ordering is crucially dependent on [r(A)] or the e(g) band w idth, and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field.