Conduction phenomenon in Mn-substituted NiCd ferrites

The dc electrical resistivity (q) and thermoelectric power (α) are studied as functions of temperature for Mn-substituted ferrites with general formula Cd0.3Ni0.7 + xMnxFe2−2xO4. At lower Mn concentrations (i.e., x 0.15) is attributed to the formation of Mn3+ clusters which lowers the concentration of Mn3+ + Fe2+ bonds. The compositional variation of thermo-e.m.f. shows n-type behaviour for the samples with x less than 0.15, whereas p-type behaviour for the samples with x > 0.15. The n-p transition is attributed to the formation of Ni3+ and Fe2+ + vacancies which act as p-type carriers. The thermo-e.m.f. remains constant for all the samples at the measured temperatures. The temperature dependences of α, Q, and mobility clearly confirm the conduction mechanism to be due to polaron hopping. The compositional variations of dielectric constant show two regions, one for x ≦ 0.15 in which the dielectric behaviour is due to Fe3+ ions and the other for x > 0.15, where the nature of the dielectric is complex. The complex dielectric structure might have its origin in the different valence states of Mn, inhomogeneities within the ferrite grain, and oxygen stoichiometry.