Appearance of a two-dimensional antiferromagnetic order in quasi-one-dimensional cobalt oxides

Abstract

By means of muon spin rotation and relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ techniques, we have investigated the magnetism of quasi-one-dimensional (1D) cobalt oxides ${A}_{n+2}{\mathrm{Co}}_{n+1}{\mathrm{O}}_{3n+3}$ ($A=\mathrm{Ca}$, Sr, and Ba; $n=1$, 2, 3, 5, and $\ensuremath{\infty}$), in which the 1D ${\mathrm{CoO}}_{3}$ chain is surrounded by six equally spaced chains forming a triangular lattice in the $ab$ plane, using polycrystalline samples, from room temperature down to 1.8 K. For the compounds with $n=1\text{--}5$, transverse field ${\ensuremath{\mu}}^{+}\mathrm{SR}$ experiments showed the existence of a magnetic transition below $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, although there were no clear anomalies in the susceptibility-vs-$T$ curve. The onset temperature of the transition $({T}_{\mathrm{c}}^{\mathrm{on}})$ was found to decrease with $n$; from 100 K for $n=1$ to 60 K for $n=5$. A damped muon spin oscillation was observed only in the sample with $n=1$ $({\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6})$, whereas only a fast relaxation obtained even at 1.8 K in the other three samples. Combining the fact that the paramagnetic Curie temperature ranges from $\ensuremath{-}150$ to $\ensuremath{-}200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the compound with $n=2$ and 3, the ${\ensuremath{\mu}}^{+}\mathrm{SR}$ result indicates that a two-dimensional (2D) short-range antiferromagnetic (AF) order, which has been thought to be unlikely to exist at high $T$ due to a relatively strong 1D ferromagnetic $(\mathrm{F})$ interaction, appears below ${T}_{\mathrm{c}}^{\mathrm{on}}$ for all compounds with $n=1\text{--}5$; but quasistatic long-range AF order formed only in ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$, below 25 K. For ${\mathrm{BaCoO}}_{3}$ $(n=\ensuremath{\infty})$, as $T$ decreased from 300 K, 1D F order appeared below 53 K, and a sharp 2D AF transition occurred at 15 K.