Effect of double exchange on thermoelectric power of CuxCoyCrzSe4

A correlation between the thermoelectric power and the double exchange and superexchange magnetic interactions in the ferromagnetically ordered CuxCoyCrzSe4 p-type spinel conductors (with low Co content y up to 0.11) and in the antiferromagnetically ordered n-type semiconductor (with Co content y = 0.23) in the temperature range from 5 to 400 K is considered, respectively. The magnetic and thermopower investigations showed that a bandwidth of the 3d t2g band due to Cr3+ and Cr4+ ions narrows with increasing cobalt content. For p-type conductors (with Co content y up to 0.11) all thermopower components are positive while for n-type semiconductor (y = 0.23) the phonon drag component is positive and the diffusion and impurity components are negative. In the latter case the linear dependence between thermopower (S) and the normalized electrical resistivity (ln ρ/ρ0) suggests a polaron conduction at high temperatures. The carrier diffusion dominates in single crystals with high cobalt content and the impurity component increases with increasing non-stoichiometry of a sample. For samples with low cobalt content y up to 0.11 the strong ferromagnetic coupling connected mainly with double-exchange mechanism makes easier the magnon excitations. Their intensities increase with increasing sample stoichiometry (i.e. when x + y + z → 3).