Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2] hexagonal layer. Both cobaltites exhibit large thermopower values (S-300 K similar to 140 muV K-1), low resistivity values (rho(300) (K) 40-60 mOmega cm) and small thermal conductivities (kappa(300) (K) similar to I W K-1 m(-1)). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = rho(H)-rho(H=0)/rho(H=0)), reaching, at 2.5 K, -85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.