Spin state transition in Ca-doped Na0.7CoO2 with the nominal Co valence below 3.16

J. Sugiyama, J. H. Brewer, E. J. Ansaldo, J. A. Chakhalian, H. Nozaki, H. Hazama, Y. Ono, T. Kajitani

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3 Scopus citations


In order to clarify the nature of the transition around 300 K in Ca-doped Na0.7CoO2, the magnetism of Na0.7Ca yCoO2 with y=0.035 and 0.07 was investigated in a positive muon spin rotation and relaxation (μ+SR) experiment. Transverse field μ+SR measurements showed that the spin state of the Co ions in Na0.7Ca0.07CoO2 changes at around 300 K; i.e. the exponential relaxation rate vs. T curve exhibited a large maximum around 300 K with an accompanying small peak in the muonic Knight shift, whereas no changes were found in the asymmetry, similar to [Ca2CoO 3]0.62[CoO2] at around 400 K. Although the spin-density-wave (SDW) state exists for NaxCoO2 with x≥0.75 at low temperatures, zero-field μ+SR spectra in the Ca-doped samples showed no muon spin precessions down to 1.8 K but only fast relaxations indicating disorder. This is probably because the Ca2+ ions in the Na planes alter the charge and/or spin distribution in the CoO 2 planes. As a result, the SDW order is hindered, as the nominal Co valence is decreased below 3.16.

Original languageEnglish (US)
Pages (from-to)36-40
Number of pages5
JournalSolid State Communications
Issue number1-2
StatePublished - Jan 2006
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Condensed Matter Physics
  • Materials Chemistry


  • A. Layered cobalt oxides
  • D. Muon spin rotation and relaxation
  • D. Spin state transition
  • D. Thermoelectrics

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