  The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare
earth,AE=Ca, Sr, Ba and Pb)have drawn considerable attention, especially
following the discovery of colossal magnetoresistance (CMR). They exhibit
extraordinary large magnetoresistance pronounced as CMR in the vicinity of
insulator-metal/paramagnetic-ferromagnetic transition at a relatively large
applied magnetic fields. However, for applied aspectes, occurence of
significant CMR at low applied magnetic fields would be required. This review
consists of of two sections: In the first section we have extensively reviewed
the salient features e.g. structure, phase diagram, double exchange mechansim,
Jahn Teller effect, different types of ordering and phase separation of CMR
mangnaites. The second is devoted to an overview of experimental results on CMR
and related magnetotransport characteristics at low magnetic fields for doped
manganites such as polycrystalline La_0.67Ca_0.33MnO_3 films, Ag admixed
La_0.67Ca_0.33MnO_3 films, polycrystalline (La_0.7Ca_0.2Ba_0.1MnO_3)and
epitaxial (La_0.67Ca_0.33MnO_3) films on different substrates, nanophasic
La_0.7Ca_0.3MnO_3, mangnaite-polymer composites (La_0.7Ba_0.2Sr_0.1MnO_3-PMMA
and La_0.67Ca_0.33MnO_3-PMMA)and double layered polycrystalline
(La_1.4Ca_1.6-xBa_xMn_2O_7) and films (La_1.4Ca_1.6Mn_2O_7). Some other
potential magnetoresistive materials e.g. pyrochlores, chalcogenides,
ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions,
nanocontacts etc have aslo been briefly dealt with. The review concludes with
the summary of results for low field magnetotransport behaviour and prospectes
for applications.
