By means of muon spin spectroscopy, we have found that K0.49CoO2 crystals undergo successive magnetic transitions from a high-T paramagnetic state to a magnetic ordered state below 60 K and then to a second ordered state below 16 K, even though K_{0.49}CoO_2 is metallic at least down to 4 K. An isotropic magnetic behavior and wide internal-field distributions suggest the formation of a commensurate helical spin density wave (SDW) state below 16 K, while a linear SDW state is likely to exist above 16 K. It was also found that K_{0.49}CoO_2 exhibits a further transition at 150 K presumably due to a change in the spin state of the Co ions. Since the T dependence of the internal-field below 60 K was similar to that for Na_{0.5}CoO_2, this suggests that magnetic order is more strongly affected by the Co valence than by the interlayer distance/interaction and/or the charge-ordering.
@article{arxiv.cond-mat/0512319,
title = {Static magnetic order in metallic K$_{0.49}$CoO$_{2}$},
author = {J. Sugiyama and H. Nozaki and Y. Ikedo and K. Mukai and J. H. Brewer and E. J. Ansaldo and G. D. Morris and D. Andreica and A. Amato and T. Fujii and A. Asamitsu},
journal= {arXiv preprint arXiv:cond-mat/0512319},
year = {2009}
}
Comments
4 pages, 5 figures, accepted for publication in Phys. Rev. Lett