Double-exchange interaction in electron-dopedCaMnO3−δperovskites

Removal of oxygen ions by thermogravimetric methods in $\mathrm{CaMn}{\mathrm{O}}_{3-\delta }$ ($0<~\delta <~0.34$) produces electron doping of the material and ${\mathrm{Mn}}^{4+}$ 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 $(3-\delta )=2.75\mathrm{} \mathrm{and} 2.84$. 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 ${\mathrm{Mn}}^{3+}$-${\mathrm{Mn}}^{4+}$ 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.