The perovskite manganites with generic formula RE(1-x)AE(x)MnO(3) (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator-metal/paramagnetic-ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g. structure, phase diagram, double-exchange mechanism, Jahn-Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

Effect of quenched disorder ͑QD͒ caused by oxygen vacancy ͑OV͒ and substrate induced inhomogeneous compressive strain, on the magnetic and transport properties of oriented polycrystalline Sm 0.55 Sr 0.45 MnO 3 thin films is investigated. QD is related intimately to the ordering/ disordering of the OVs and controls the paramagnetic-ferromagnetic/insulator-metal transition. OV ordered films show enhanced T C / T IM ϳ 165 K, which is depressed by oxygen annealing. OV disordering realized by quenching reduces T C / T IM . The first order IM transition observed in SSMO single crystals is transformed into nonhysteretic and continuous one in the OV ordered films. QD appears to be diluted by OV disorder/annihilation and results in stronger carrier localization.

Calcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method-whose main advantages are versatility, high reproducibility and scalability-yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with.

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The magnetoresistive behaviour of La 0.67 Ca 0.33 MnO 3 thin films deposited by ultrasonic spray pyrolysis on YSZ substrate has been investigated at low applied dc magnetic fields. The highest magneto-resistance ratio (MR) of −17.75% has been observed at 77 K for applied dc magnetic field, H dc = 3 kG. The variation of the MR with temperature has been investigated and it has been observed that at lower temperatures the increase in MR is very rapid. This MR-T behaviour shows that the magneto-resistance has its origin in the grain boundaries. The variation of the MR with H dc shows a faster increase in the MR at lower applied dc fields. Surface morphological explorations employing scanning electron microscopy show the presence of microcracks throughout the films surface and also that these films are micro-crystalline, having an average grain size of ∼125 nm.