Giant Magnetoresistance Phenomenon in Manganates

The discovery of giant magnetoresistance (GMR) in rare earth manganates of the general formula Ln 1–x A x MnO 3 (Lnrare earth, A=divalent cation) has aroused much interest not only because of its technological implications, but also due to the fascinating features and mechanism of the phenomemon in these oxides. GMR is observed in these manganates when they become ferromagnetic and transform from an insulating state to a metallic state close to the Curie temperature. The essential features of magnetoresistance in the manganates can be understood on the basis of the double‐exchange mechanism, but this is too simplistic to account for all the observed data. The most curious property of the manganates relates to the high resistivity exhibited in the so‐called metallic state. Charge ordering competes with the double‐exchange interaction responsible for ferromagnetism and GMR in these materials. The charge‐ordered (charge‐crystal) insulating state in the rare earth manganates can be melted into a metallic and ferromagnetic charge‐liquid state by applying a magnetic field, thus providing a unique case of charge and spin separation in solids. The observation of GMR in Tl 2 Mn 2 O 7 shows that there can be causes other than double‐exchange for the phenomenon.