Magnetism of layered cobalt oxides investigated by muon spin rotation and relaxation

Muon spin rotation-relaxation (muSR) spectroscopy has been used to investigate the magnetic properties of polycrystalline Ca3-xMxCo4O9 (xless than or equal to0.5,M=Sr,Y, and Bi) and Na0.7CoO2 samples in the temperature range between 2.5 and 300 K. It was found that Ca3Co4O9 exhibits a magnetic transition at around T-c=100 K; at lower temperatures, two types of relaxation were observed using a weak transverse field muSR technique with H=104 Oe: one with a fast relaxation rate lambda(F)similar to10 mus(-1) and the other with a slow lambda(S)similar to0.1 mus(-1). Zero-field muSR measurements suggest the existence of an incommensurate spin-density-wave (SDW) state below T-c (i.e., T-c=T-SDW), although a ferrimagnetic M-H loop was observed by a dc susceptibility measurement below 19 K. The substitution of Y or Bi for Ca increased T-SDW, while the substitution of Sr for Ca did not affect T-SDW. This indicates that the SDW transition depends strongly on the average valence of the Co ions. The related material Na0.7CoO2 showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of Ca3Co4O9 and Na0.7CoO2, we suggest that Co ions in the rocksalt-type layers of Ca3Co4O9 are likely to play a significant role in inducing the SDW transition around 100 K.