Improving the spin entropy by suppressing Co4+ concentration in thermoelectric Ca3Co4O9+δ

We report the effects of rare earth Gd doping on the spin entropy in Ca3Co4O9+δ. Magnetothermopower is found to be strongly magnetic-field dependent and increases upon Gd doping, revealing that Gd doping can improve the spin entropy of Ca3Co4O9+δ. X-ray photoemission spectroscopy (XPS) results allow us to determine the decrease in Co4+ concentration induced by Gd doping. Our results confirm that the spin-entropy enhancement originates from the decrease of Co4+ concentration. The present study suggests that we can improve the spin entropy by suppressing Co4+ concentration in layered cobalt oxides.