Colossal magnetoresistance perovskites

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... 

Colossal magnetoresistance (CMR) is observed below 300 K in manganese perovskite structures [7] this has not been used in technology. 

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. 

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. 

Perovskites have also received much attention since 1986 because the superconducting oxide YBCO contains perovskite structural elements. The importance of this structure was again realized in 1993 when the phenomenon of colossal magnetoresistance (CMR) was discovered in a range of manganate ceramics with a layered perovskite structure similar to that found in YBCO and other high-temperature superconductors. 

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. 

For a review of correlated electron systems, including perovskite based superconductors, colossal magnetoresistance, manganites and cobaltites see ... 

Binat7 oxides with the stoichiometry ABO3 often have the perovskite structure. The versatility of these oxides is mostly due to the multivalent and multispin metal ions that may occupy site B of the unit cell (Fig. 19.1) and combined with some covalent character of the metal—oxygen bonds, the electric and magnetic properties of these oxides become unique. A large family of these oxides shows colossal magnetoresistance (CMR). 

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