Colossal magnetoresistive manganites

Magnetoelectronic features of the perovskite-type manganites are overviewed in the light of the mechanism of the colossal magnetoresistance (CMR). The essential ingredient of the CMR physics is not only the double-exchange interaction but also other competing interactions, such as ferromagnetic/antiferromagnetic superexchange interactions and charge/orbital ordering instabilities as well as their strong coupling with the lattice deformation. In particular, the orbital degree of freedom of the conduction electrons in the near-degenerate 3d e(g) state plays an essential role in producing the unconventional metal-insulator phenomena in the manganites via strong coupling with spin, charge, and lattice degrees of freedom. Insulating or poorly conducting states arise from the long or short-range correlations of charge and orbital, but can be mostly melted or turned into the orbital-disordered conducting state by application of a magnetic field, producing the CMR or the insulator-metal transition. (C) 1999 Elsevier Science B.V. All rights reserved.