Effects of Co doping in bilayered manganite LaSr2Mn2O7:: Resistivity, thermoelectric power, and thermal conductivity -: art. no. 184417

Systematic studies of resistivity, thermoelectric power, and thermal conductivity have been performed on polycrystalline bilayered manganites LaSr2Mn2-xCoxO7 (0 <= x <= 0.2). As x <= 0.04, it is found that the temperature dependence of both Seebeck coefficient S(T) and resistivity rho(T) in the high-temperature region follows the small polaron transport mechanism. However, in the low-temperature region, variable-range-hopping model matches the experimental data better. As x >= 0.08, the very large magnitude of Seebeck coefficient with over 100 mu V/K at low temperatures is observed, which is suggested to originate from the spin state transition of Co3+ ions from the intermediate-spin (IS) state or high-spin (HS) state to low-spin (LS) state and the unbalance of the spin and orbital degrees of freedom between Co3+ and Co4+ sites. In addition, the variable sign in the S(T) data is observed at low temperatures for the samples with Co-doping level (x >= 0.04), which is suggested to be related to the narrowing of accompanying band. As to thermal conduction k(T), the low-temperature peak is suppressed due to Co doping. The variation of thermal conduction is analyzed based on the combined effect due to the suppression of local Mn3+O6 Jahn-Teller (JT) lattice distortion because of the substitution of non-JT Co3+ ions with LS and HS states for JT Mn3+ ions, which results in the increase of thermal conduction, and the introduction of the disorder due to Co doping, which contributes to the decrease of thermal conduction.