Abstract
The ground states of La1-xCaxMnO3 change from double-exchange-associated conducting ferromagnets at x<0.50 to charge-ordered antiferromagnets at x>0.50 even when x is varied only slightly around 0.50. We have performed a careful study of the electrical and magnetic properties of these materials for a range of samples with slightly varying calcium doping around x0.50. The natures of the ground state and the transition were found to be sensitive not only to the calcium doping but also to the actual Mn4+ content of these materials. The low-temperature resistivity of the x0.50 materials below the charge-ordering temperature was found to drop with decreasing temperature in the presence of magnetic fields of even a few teslas or a slight Ca underdoping (x0.50), revealing the presence of some free carriers even in the charge-ordered states. The data therefore demonstrate a coexistence of ferromagnetic conducting and antiferromagnetic charge-ordered phases in the perovskite manganites. We also observe field-induced `annealing' in the charge-ordered state due to the partial delocalization of the carriers.
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