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

Giant magnetoresistance (GMR) and related properties of manganate perovskites of the general formula Lnl − xAxMnO3 (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 Ln1 − xAxMnO3 compositions exhibit charge-ordering and related 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, charge-ordering and double-exchange being competing interactions.