Metal-semiconductor and magnetic transitions in compensated polycrystalline La1-xCaxMnO3-δ (x=0.20,0.25) -: art. no. 134448

The structures, magnetizations, conductivities, and Seebeck coefficients of single-phase polycrystalline La0.75Ca0.25MnO3-delta and La0.80Ca0.20MnO3-delta samples have been investigated as a function of the oxygen-vacancy concentration. Electrons from oxygen vacancies compensate holes introduced by Ca cations. Thus, by altering the oxygen deficiency delta the carrier concentration is changed while the calcium concentration remains constant. The effects of carrier and calcium concentrations on transport and magnetism are thereby separated from one another. We find the conductivity to be very sensitive to oxygen vacancies while the magnetization is not. In particular, oxygen vacancies decrease the conductivity and can eliminate low-temperature metal-like conduction. Thus ferromagnetism in La0.75Ca0.25MnO3-delta and La0.80Ca0.20MnO3-delta can occur without the concomitant occurrence of metallic conduction. However, electronic properties are sensitive to magnetic order. In particular, the temperature dependence of the Seebeck coefficient changes at the Curie temperature. These results are consistent with Jonker's suggestion that ferromagnetism in these manganites is induced by dopant-induced strains rather than by dopant-induced charge carriers. Loss of magnetic order fosters mobile charge carriers' collapse to localized dopant-related polaron states. Thus, the magnetically induced metal-semiconductor transitions in doped manganites are analogous to those of suitably doped ferromagnetic EuO.