Unique Behavior of Seebeck Coefficient in N-Type CdCr2Se4

Abstract

Electrical resistivities ρ, normal Hall coefficients R₀, and Seebeck coefficients Q, are reported for a series of samples of the semiconducting ferromagnetic compound CdCr₂Se₄, doped with varying amounts of indium. The behavior of ρ and of R₀ indicates that from room temperature down to 150°K the electrons deionize onto the donor centers. From 150°K down to about 55°K, the carrier concentration increases by approximately four orders of magnitude, accompanied by a proportional drop in ρ; this can be attributed to the known decrease in bandgap with lowering of T, if the donor level is assumed to remain constant with reference to the valence‐band edge. The rise of ρ below 55°K is ascribed to impurity conduction. The mobilities reach a maximum near T_c, and fall off on both sides. Strong dependence of R₀ on the magnetic field H is found above 110°K. The magnetoresistance is negative, and peaks sharply at 130°K; it obeys different power laws on the two sides of its peak. The Seebeck coefficient Q shows unique dependence on T, H and on In content: it displays local extrema at 150°K and at 125°K. Between these two temperatures, H has a strong effect on Q, depending on the In concentration. No simple relation between the behavior of Q on the one hand, and of ρ and R₀ on the other, has been deduced. The model suggested by the data requires two bands, with charge carriers whose concentrations and mobilities are very different and have strong dependencies on T.