A study of the magnetic and electrical crossover region of La_0.5±δCa_0.5∓δMnO₃

The ground states of La_1-xCa_xMnO₃ 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 x ≈ 0.50. 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 x ≈ 0.50 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 (x ≲ 0.50), 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.