COMPETITION BETWEEN CHARGE ORDERING AND FERROMAGNETISM IN MANGANESE PEROVSKITES

Manganese perovskites have recently been the subject of renewed attent ion, due to the ''colossal'' magnetoresistance (CMR) and the coupled m etal-insulator and magnetic transitions displayed at the spin-ordering temperature T-C by some of these compounds. The charge localization m echanism above T,appears at present to be the single most important is sue in manganite research. Neutron and synchrotron X-ray diffraction s tudies have revealed, for appropriate doping ranges, a strong competit ion between the metallic state, which is ferromagnetic and shows small lattice distortions, and a charge-localized/ordered state that become s antiferromagnetic at low temperatures and shows large lattice distor tions of the Jahn-Teller type. In the case of La0.5Ca0.5MnO3 and Pr0.7 Ca0.3MnO3, these two states can coexist giving rise to transient inhom ogeneities. The possibility that a similar situation may be reproduced in the paramagnetic phase will be discussed, in the light of recent n eutron-diffuse-scattering data suggestive of extended ferromagnetic fl uctuations aboveT(C). (C) 1998 Elsevier Science B.V. All rights reserv ed.