Giant magnetoresistance and related properties of rare-earth manganates and other oxide systems

Abstract

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 Ln_1−xA_xMnO₃ (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 e_g band width, and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field.