Effect of co-doping of Na+ and Bi3+ ions on magnetotransport and thermopower studies of La0.67Sr0.33MnO3 manganite

The current work discusses on the effect of sodium (Na) and bismuth (Bi) ion doping at different concentrations in the compositional formula La_0.67Sr_0.33–yNa_y/2Bi_y/2MnO₃ (y = 0, 0.0825, 0.165, 0.2475, 0.33) on structural, magnetic, magnetotransport and thermoelectric studies. X-ray diffraction confirms the R\(\:\stackrel{-}{3}\)c space group with rhombohedral structure and the field emission scanning electron microscope images confirms a systematic growth of grains with increasing in NaBi concentrations. An enhancement in the ferromagnetic ordering and magnetic transitions above 325 K were observed up to y = 0.165 and they decrease thereafter. A broad maximum along with a double peak behavior were noticed in the temperature dependent resistivity curves and they disappear upon the application of external magnetic fields. The Seebeck coefficient show a change of sign from positive to negative with increasing temperature for lower concentrations of NaBi and they exhibit a negative sign for higher substitutions suggesting the variation in the nature of charge carriers. The small polaron hopping mechanism governs the resistivity and thermopower behavior in the high temperature regions while, electron-electron, electron-magnon, magnon-magnon scattering mechanism contributes in the low temperature region.