The magnetic, electrical and thermal transport studies in the layered cobalt oxide Nd1−xSr1+xCoO4 (x = 0.25 and 0.33)

Abstract

The magnetization, resistivity ρ, thermoelectric power S, thermal conductivity κ and specific heat Cp in the layered cobalt oxide Nd1−xSr1+xCoO4 (x = 0.25 and 0.33) have been investigated systematically. Based on the temperature dependence of susceptibility χm(T) and Seebeck coefficient S(T), a combination of the low-spin state for Co4+ and the intermediate-spin (IS) state for Co3+ can be suggested. The very large magnitude of S is suggested to originate from the configurational entropy of charge carriers enhanced by their spin and orbital degeneracy between Co3+ and Co4+ sites. In addition, based on the analysis of the temperature dependence of S(T) and ρ(T), the high temperature small polaron conduction and the low temperature variable range hopping conduction are suggested, respectively. As to thermal conduction κ(T), the decrease in κ with increasing Sr-doping is suggested to come from the combined effect of the scattering of the Co3+O6 Jahn–Teller distortion with the IS state and the disorder to the phonon. Meanwhile, in the specific heat Cp(T) curves, the samples containing magnetic rare-earth Nd3+ ions reveal an obvious Schottky-type anomaly due to the crystal-field effects.