Transport elektronowy wymuszony gradientem temperatury w wybranych spinelach chromowych

Abstract

The problems of the electronic transport forced by the temperature gradient in the selected chromium spinels: ZnCr2 – xNixSe4, Zn1 – xCuxCr2Se4, ZnxCuyCrzSe4, CuxGayCrzSe4, CdCr2 – xSbxSe4, and Hg1 – xCuxCr2Se4 in the frames of the theories of Mott, Debye and Bloch have been discussed in this work. The analysis carried on of the temperature dependencies of thermopower showed that its resultant value consists of the following components: diffuse component, phonon drag, magnon drag and the impurity component. It was observed in all the systems under study that the diffuse component is the dominant component of the thermopower. In the temperature range under study its value does not exceed 100 μV/K and it is greater of one order of magnitude as compared with the classical metals. As it follows from the studies, a close correlation exists between the magnetic interactions realized by the mechanism of the double exchange and the appearance of the component of the thermopower related to the spin wave excitations. The RKKY interactions seem to be the factor which supports the effect of the coherent transfer of magnon momenta to the current carriers. It was observed that the defectiveness of spinel structure makes a factor which also substantially influences the value of the intensity of the magnetic component of the thermopower. In the systems, where a large defectiveness occurs, one observes the decrease of the value of the thermopower and the shift of its maximum in the direction of the lower temperatures. The second factor, which causes the substantial increase of the value of the thermopower, is the effect related to the phonon drag. The obtained results suggest that the nonstoichiometry of the system neither affects very much the value of the phonon component, nor the position of its temperature maximum. The analysis of thermopower revealed also that in the systems, in which the double exchange mechanism dominates, one has to deal with the electronic transfer in the mixed valence band of the chromium ions. Moreover the lack of this mechanism confirmed by the XANES and EXAFS studies reveals the electronic transport related either to the carrier hopping in the forbidden gap or to the distorsion of the tetrahedral and octahedral positions caused by the polarons. The studies presented by the author prove that the kind of the magnetic structure, the nonstoichiometry as well as the defectiveness of the crystal lattice strongly influence the electronic transport forced by the temperature gradient in the materials with the spinel structure.