Double-exchange interaction in electron-doped CaMnO3-delta perovskites

Removal of oxygen ions by thermogravimetric methods in CaMnO3-delta (0 less than or equal to delta less than or equal to 0.34) produces electron doping of the material and Mn4+ ions are reduced to a trivalent state. Electrical resistivity measurements in samples with controlled oxygen content show a strong increase of the conductivity for samples with (3-delta)=2.75 and 2.84. The paramagnetic susceptibility at high temperatures can be described by a Curie-Weiss law with unusually large values for the Curie constant, which is also dependent on the oxygen stoichiometry. These results are interpreted in terms of ferromagnetic Mn3+-Mn4+ spin clusters formed around the oxygen vacancies. An extension of the Anderson-Hasegawa Hamiltonian for the double-exchange interaction was made in order to understand the electric and magnetic behavior of these materials.