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

Muon spin rotation-relaxation (μSR) spectroscopy has been used to investigate the magnetic properties of polycrystalline Ca_3-xM_xCo₄O₉ (x<~0.5,M=Sr,Y, and Bi) and Na_0.7CoO₂ samples in the temperature range between 2.5 and 300 K. It was found that Ca₃Co₄O₉ exhibits a magnetic transition at around T_c=100K; at lower temperatures, two types of relaxation were observed using a weak transverse field μSR technique with H=104Oe: one with a fast relaxation rate λ_F∼10μs^-1 and the other with a slow λ_S∼0.1μs^-1. Zero-field μSR 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 Na_0.7CoO₂ showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of Ca₃Co₄O₉ and Na_0.7CoO₂, 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.