Impact of Charge Ordering on Magnetic Correlations in Perovskite(Bi,Ca)MnO3

Single crystalline ${\mathrm{Bi}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ ( $0.74\le x\le 0.82$) 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.