Influence of nonstoichiometry on the lattice excitations in the Zn x Cu y Cr z Se4 p-type spinel ferromagnetic conductors

The thermoelectric power analysis was used to study an influence of nonstoichiometry on the lattice excitations in the ferromagnetically ordered Zn x Cu y Cr z Se4 p-type spinel conductors in the temperature range from 5 to 400 K between y=0.81 and 0.97. The above investigations showed that the thermoelectric power can be well explained considering the diffusion, phonon drag, magnon drag and impurity components. With increasing stoichiometry of a single crystal (i.e., with x+y+z→3): diffusion and impurity components decrease, phonon drag and magnon drag components increase and the phonon and magnon drag peaks shift into higher temperatures and their intensity increase. Additionally, the thermoelectric power considerations showed that the strong ferromagnetic coupling connected mainly with double-exchange and RKKY mechanisms in the spinels under study make easier the spin wave excitations on one side and the nonstoichiometry hinders the spin wave excitations on the other. These effects are interpreted within a framework of the Mott, Debye and the Bloch theories