Magnetic nature ofKxCoO2near the antiferromagnetic phase withx=0.5: Positive muon spin rotation and relaxation

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In order to elucidate the magnetic nature of ${\mathrm{K}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ in the vicinity of $x=0.5$, we have measured positive muon spin rotation and relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ spectra using polycrystalline samples of $x=0.6$, $4∕7$, and 0.5 in the temperature range between 1.8 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A zero-field spectrum suggests the existence of localized but disordered moments below $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the $x=0.6$ sample, whereas the spectra reveal the absence of magnetic moments in the $x=4∕7$ sample. Combining our ${\ensuremath{\mu}}^{+}\mathrm{SR}$ results with the results of resistivity and susceptibility measurements, it is found that the phase boundary between the Curie-Weiss metal and Pauli-paramagnetic metal exists between $x=0.6$ and $4∕7$. For the rhombohedral $x=0.5$ sample (i.e., $\ensuremath{\beta}$ phase), the quasistatic antiferromagnetic (AF) order appears below $58\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ $(={T}_{N})$, and the whole sample enters into the AF phase below ${T}_{N}$, as in the case for the hexagonal $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{K}}_{0.5}\mathrm{Co}{\mathrm{O}}_{2}$. This suggests that the interplane interaction is most unlikely to be crucial for determining the magnetic nature of the $\mathrm{Co}{\mathrm{O}}_{2}$ plane. The complex $T$ dependence of the internal AF fields, particularly the drastic change at the charge-order transition temperature $({T}_{\mathrm{C}\mathrm{O}}=20\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, is qualitatively explained by the change in the muon sites induced by charge order in the $\mathrm{Co}{\mathrm{O}}_{2}$ planes.