Ferromagnetism in the (Bi, Sr)MnO₃ Solid Solution Synthesized under High Pressure

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  BiMnO₃ synthesized at high pressure is a triclinically distorted perovskite and a ferromagnetic insulator with T_c = 105 K, in contrast to antiferromagnetic LaMnO₃. Though the conductivity of Bi_1-xSr_xMnO₃ increased with x, no metallic state was achieved to the solid solution limit of x = 0.67. Furthetmore saturation magnetic moment of 3.6 μ_B observed in BiMnO₃ decreased very rapidly with increasing x, and ferromagnetism disappeared for x > 0.4. These observations suggest that the highly polarizable Bi³⁺ ion with 6s² lone pair would cause local distortion of the perovskite lattice, which presumably reduces the mobility of carriers. The ferromagnetism in BiMnO₃ cannot be compatible with the double exchange mechanism, but is probably related to the orbital ordering of manganese ions in the perovskite lattice. The ferromagnetic moment of Bi_1-xCa_xMnO₃ appeared in Ca-rich composition of 0.825 < x < 0.95, showing maximum value of 1.1 μ_B per a Mn atom at x = 0.875. The electrical resistivity remarkably decreases with the Bi substitution, but never shows metallic behavior. The ferromagnetism achieved by electron doping seems to be suppressed by charge ordering with further doping.