Spin-glass transition and giant paramagnetism in heavily hole-doped Bi ₂Sr₂Co₂O_y

Hole-doped single crystals of misfit-layered cobaltate Bi _2-xPb_xSr_2-zCo₂O_y (x = 0-0.61, y = 8.28-8.62, and z = 0.01-0.22) have been successfully grown using the optical floating-zone method. Heavier hole doping has been achieved through both Pb substitution in the Bi site and the more effective Sr vacancy formation. The Co⁴⁺ : Co³⁺ ratio can be raised significantly from its original ∼1 : 1 to 4.5 : 1, as confirmed by iodometric titration. A spin-glass transition temperature of Tg ∼ 70K is confirmed by ac susceptibility measurement when the Co⁴⁺ : Co³⁺ ratio becomes higher than 2 : 1, presumably owing to the significantly increased probability of triangular geometrical frustration among antiferromagnetically coupled localized Co⁴⁺ spins.