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 ∼ 140 µV K⁻¹), low resistivity values (ρ_300 K = 40–60 mΩ cm) and small thermal conductivities (κ_300 K ∼ 1 W K⁻¹ m⁻¹). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = ρ_H − ρ_{H = 0}/ρ_{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.