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

In order to clarify the nature of the transition around 300 K in Ca-doped Na0.7CoO2, the magnetism of Na0.7CayCoO2 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 [Ca2CoO3]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 CoO2 planes. As a result, the SDW order is hindered, as the nominal Co valence is decreased below 3.16.