Colossal magnetoresistance effect

The discovery in 1986 of high-temperature superconductivity in ceramic cuprates of perovskite structure started a period of very intensive research of transition metal oxides. Soon afterwards, in 1993, the colossal magnetoresistance effect was discovered in manganite perovskites, again leading to an increasing research activity in the field of magnetic oxides. It is... 

The electrical transport properties of rare earth manganites with perovskite-type structure have been extensively studied in recent years because of the colossal magnetoresistance (CMR) effect or potential applications as catalysts. In most cases the general formula of rare earth manganites used in these studies are Ln,, (A,(Mn03, where A is a divalent ion (A = Ba, Sr, Ca, Pb) substituting for La. 

Excluding the cuprate superconductors, manganese per-ovskite oxides that exhibit the so-called colossal magnetoresistance (CMR) effect constitute, probably, the most extensively studied set of materials over the past decade or so. Several review articles exist. ... 

Colossal magnetoresistance studies of ceramics have demonstrated no significant influence of ceramic density on the magnetoresistance at H = 1.5T. However, ceramics obtained by the solid state synthesis method and containing inclusions of lanthanum-doped manganese oxide displayed substantially higher magnetoresistance values. This effect was later used for directed enhancement of low field... 

Abstract A review is made on the developments in the last two decades in the held of the Jahn-Teller effect on itinerant electrons in Jahn-Teller crystals. Special attention is paid to the current status of the researches on the fullerene superconductors and the manganite perovskites exhibiting the colossal magnetoresistance. Present knowledge about the polarons and bipolarons in the typical Jahn-Teller model systems is also summarized, together with some original results of our own. 

In this chapter the focus is upon electronic conductivity in perovskites. The electrons in perovskites are believed to be strongly correlated that is, they do not behave as a classical electron gas, but are the subject to electron-electron interactions. This leads to considerable modification of the collective electron behaviour of the conduction electrons, resulting in metal-insulator transitions, high-temperature superconductivity, half-metals and colossal magnetoresistance (CMR). The effects of strong correlation are important for the 3d, 4d and4f elements. In many ways the topics described here are thus a continuation of the previous chapter on magnetic perovskites, and in truth the two subject areas cannot be separated in a hard and fast maimer. 

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