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Static magnetic order in Na$_{0.75}$CoO$_2$ detected by muon spin rotation and relaxation

Jun Sugiyama·Hiroshi Itahara·Jess H. Brewer·Eduardo J. Ansaldo·Teruki Motohashi·Maarit Karppinen·Hisao Yamauchi
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Abstract

The nature of the magnetic transition of the Na-rich thermoelectric Na$_{0.75}$CoO$_2$ at 22K was studied by positive muon-spin-rotation and relaxation ($μ^+$SR) spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero field $μ$SR measurements indicated the existence of a static internal magnetic field at temperatures below 22 K (= $T_{\rm m}$). The observed muon spin precession signal below $T_{\rm m}$ consisted of three components with different precession frequencies, corresponding to three inequivalent muon$^+$ sites in the Na$_{0.75}$CoO$_2$ lattice. The total volume fraction of the three components was estimated as $\sim$21% at 2.5 K; thus, this magnetic transition was not induced by impurities but is an intrinsic change in the magnetism of the sample, although the sample was magnetically inhomogeneous otherwise. On the other hand, a similar experiment on a Na$_{0.65}$CoO$_2$ sample exhibited no magnetic transition down to 2.5 K; which indicates that the average valence of the Co ions is responsible for inducing the magnetic transition at 22 K.

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Jun Sugiyama, Hiroshi Itahara, Jess H. Brewer, Eduardo J. Ansaldo, Teruki Motohashi, Maarit Karppinen, Hisao Yamauchi. 2003-05-13. Static magnetic order in Na$_{0.75}$CoO$_2$ detected by muon spin rotation and relaxation. https://doi.org/10.1103/physrevb.67.214420

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