COLOSSAL MAGNETORESISTANCE, CHARGE ORDERING AND OTHER NOVEL PROPERTIES OF MANGANATES AND RELATED MATERIALS

Colossal magnetoresistance (CMR) and related properties of perovskite manganates of the general formula Ln_1−xA_xMnO₃ (Ln = rare-earth; A = divalention) are discussed in detail. 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 commonly attributed to double-exchange. CMR 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. Recent studies clearly indicate that double-exchange alone cannot explain the many fascinating features of the manganates. Some of the Ln_l−xA_xMnO₃ compositions exhibit charge-ordering and related effects. Charge ordering is crucially dependent on 〈r_A〉 or the e_g bandwidth 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. The importance of Jahn-Teller interaction in determining the various properties of manganates is highlighted. CMR found in a few inorganic solids other than the perovskite manganates is pointed out.