Impact of charge ordering on magnetic correlations in perovskite (Bi,Ca)MnO3

Single crystalline Bi1-xCaxMnO3 (0.74 less than or equal to x less than or equal to 0.82) were studied with neutron scattering, electron diffraction, and bulk magnetic measurement. We discovered dynamic ferromagnetic spin correlations at high temperatures, which are replaced by antiferromagnetic spin fluctuations at a concomitant charge ordering and structural transition. Our results indicate that thermal-activated hopping of the Jahn-Teller active e(g) electrons in these insulating materials, nevertheless, induces ferromagnetic interaction through; the double-exchange mechanism. It is the ordering of these charges which competes with the double-exchange ferromagnetic metallic state.