Metal–insulator transitions and giant magnetoresistance in perovskite oxides

Transition–metal oxides of perovskite structure exhibit compositionally controlledmetal–insulator (M–I) transitions wherein the temperature coefficient of resistivity changes sign at a resistivity value close to Mott's maximum metallic resistivity. These oxides also obey the Mott's criterion, nc1/3aH ≈ 0.25, arising from electron interactions. Cuprate superconductors, in spite of their anomalous properties in the metallic state, obey the above two criteria for metallicity. The cuprates also exhibit unusual insulator–superconductor and M–I transitions. Perovskite manganates which exhibit giant magnetoresistance, on the other hand, show high resistivities (ρ>ρmax) of Mott) at low temperatures, in the ‘metallic’ state, and the mechanism of the M–I transition in these materials is not clear. Jahn–Teller distortion and charge ordering compete with double exchange and the electronic properties are accordingly determined by the dominant interaction. While double exchange favours ferromagnetism and metallicity, charge–ordering and Jahn–Teller effects favour insulating behaviour. Charge ordering in the manganates is specially interesting being strongly affected by the size of the A–site cations.