Size effect on the colossal thermoelectric power in charge ordered small band width manganites based on Gd-Sr

Earlier we observed colossal thermoelectric power in charge ordered intermediate band width manganite La_0.5Ca_0.5MnO₃ and was explained based on charge ordering and occurrence of spin glass states. With a view to extending such a study on small band width Gd-Sr manganites, both unmilled and milled forms of Gd_1−xSr_xMnO₃ (x = 0.3, 0.5, and 0.6) samples were prepared. Nano forms of Gd_1−xSr_xMnO₃ were prepared by high energy ball milling. All compositions, both milled and unmilled forms, exhibited colossal thermoelectric power. The absolute value of thermoelectric power almost doubled in the case of milled samples and attained a maximum value of −69 mV K⁻¹ at 42 K, where a spin glass transition takes place. In order to explain the occurrence of transition at ∼42 K, field cooling and zero-field cooling magnetic measurements were conducted and we found that the peak value of thermoelectric power is observed at the same magnetic ordering temperature. The results are further modelled using Mandal's model by incorporating Kondo properties of spin glass along with magnon scattering. (paper)