Phase separation in theCoO2layer observed in thermoelectric layered cobalt dioxides

Abstract

$^{59}\text{C}\text{o}$ nuclear magnetic resonance (NMR) measurements have been performed to study the local magnetic properties of the misfit layered cobalt dioxides (MLCO's) with the ${\text{CoO}}_{2}$ and rock-salt layers, ${[{\text{Ca}}_{2}{\text{CoO}}_{3}]}_{0.62}{\text{CoO}}_{2}$ $(\ensuremath{\equiv}{\text{Ca}}_{3}{\text{Co}}_{3.92}{\text{O}}_{9.34})$ and ${\text{Ca}}_{3}{\text{Co}}_{3.92}{\text{O}}_{9.34\ensuremath{-}\ensuremath{\delta}}$ with oxygen nonstoichiometry. The $^{59}\text{C}\text{o}$ NMR spectrum consists of mainly five lines at 4.2 K at which the samples are in a magnetically ordered state. Among the five NMR lines for ${\text{Ca}}_{3}{\text{Co}}_{3.92}{\text{O}}_{9.34}$, three lines at higher frequencies ($f$'s) satisfy the resonance condition with two branches indicating the presence of antiferromagnetic internal fields (${H}_{\text{int}}$'s). The other two lines exhibit one branch, and one of the two has a nonzero ${H}_{\text{int}}$ under zero external field (ZF), which signifies the existence of ferromagnetic (FM) ${H}_{\text{int}}$'s. The other has a zero ${H}_{\text{int}}$ under ZF. By taking account of both the valence state of the Co ions in each layer and the lattice modulation due to the misfit between the ${\text{CoO}}_{2}$ layer and the rock-salt layer, the NMR spectra at higher $f$'s are attributed to the Co in the rock-salt layer, whereas those at lower $f$'s to the Co in the ${\text{CoO}}_{2}$ layer. Furthermore, a spin-density wave order appears to coexist with a FM order in the ${\text{CoO}}_{2}$ layer for MLCO's. The magnetic and transport properties of these materials are discussed in terms of a separation between two phases.