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

Abstract

Muon spin rotation-relaxation $(\ensuremath{\mu}\mathrm{SR})$ spectroscopy has been used to investigate the magnetic properties of polycrystalline ${\mathrm{Ca}}_{3\ensuremath{-}x}{\mathrm{M}}_{x}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ $(x<~0.5,M=\mathrm{Sr},\mathrm{Y},$ and Bi) and ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2}$ samples in the temperature range between 2.5 and 300 K. It was found that ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ exhibits a magnetic transition at around ${T}_{c}=100\mathrm{K};$ at lower temperatures, two types of relaxation were observed using a weak transverse field $\ensuremath{\mu}\mathrm{SR}$ technique with $H=104\mathrm{Oe}:$ one with a fast relaxation rate ${\ensuremath{\lambda}}_{F}\ensuremath{\sim}10\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$ and the other with a slow ${\ensuremath{\lambda}}_{S}\ensuremath{\sim}0.1\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}.$ Zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements suggest the existence of an incommensurate spin-density-wave (SDW) state below ${T}_{c}$ (i.e., ${T}_{c}{=T}_{\mathrm{SDW}}),$ although a ferrimagnetic $M\ensuremath{-}H$ loop was observed by a dc susceptibility measurement below 19 K. The substitution of Y or Bi for Ca increased ${T}_{\mathrm{SDW}},$ while the substitution of Sr for Ca did not affect ${T}_{\mathrm{SDW}}.$ This indicates that the SDW transition depends strongly on the average valence of the Co ions. The related material ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2}$ showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ and ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2},$ we suggest that Co ions in the rocksalt-type layers of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ are likely to play a significant role in inducing the SDW transition around 100 K.