Abstract
Manganese perovskites have recently been the subject of renewed attention, due to the “colossal” magnetoresistance (CMR) and the coupled metal-insulator and magnetic transitions displayed at the spin-ordering temperature
T
C by some of these compounds. The charge localization mechanism above
T
c appears at present to be the single most important issue in manganite research. Neutron and synchrotron X-ray diffraction studies have revealed, for appropriate doping ranges, a strong competition between the metallic state, which is ferromagnetic and shows small lattice distortions, and a charge-localized/ordered state that becomes antiferromagnetic at low temperatures and shows large lattice distortions of the Jahn-Teller type. In the case of La
0.5Ca
0.5MnO
3 and Pr
0.7Ca
0.3MnO
3, these two states can coexist giving rise to transient inhomogeneities. The possibility that a similar situation may be reproduced in the paramagnetic phase will be discussed, in the light of recent neutron-diffuse-scattering data suggestive of extended ferromagnetic fluctuations above
T
C.