Abstract
In order to clarify the nature of the transition around 300
K in Ca-doped Na
0.7CoO
2, the magnetism of Na
0.7Ca
y
CoO
2 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 Na
0.7Ca
0.07CoO
2 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 [Ca
2CoO
3]
0.62[CoO
2] at around 400
K.
Although the spin-density-wave (SDW) state exists for Na
x
CoO
2 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 Ca
2+ 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.