Thermoelectric power of potassium doped lanthanum manganites at low temperatures

Abstract

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La 1− x K x MnO 3 polycrystalline pellets ( x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature ( T C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region ( T> T C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region ( T< T C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T> T C whereas at low temperatures ( T< T C) in ferromagnetic region ( S FM), S FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.