Giant magnetoresistance, charge ordering and other novel properties of perovskite manganates

Abstract

Giant magnetoresistance (GMR) and related properties of manganate perovskites of the general formula Ln_l−xA_xMnO₃ (Ln=rare earth; A=divalent ion) are discussed in detail. There is a fine interplay of magnetic exchange, structural properties and electronic transport in these materials which gives rise to several novel properties. The 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. This behavior is attributed to double-exchange. 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> or pressure. The commonalities and correlations found in the properties of manganates are examined along with certain unusual features in the electron-transport properties of these materials. Some of the Ln_1−xA_xMnO₃ compositions exhibit charge-ordering and related 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, charge-ordering and double-exchange being competing interactions.