Muon spin relaxation study of misfit-layered cobalt dioxide

In order to clarify the nature of a possible magnetic phase transition at Tm=13 K for the misfit layered cobalt dioxide with a double rocksalt (RS-) type block, [Ca0.85OH]1.16RS[CoO2], we have measured the muon spin rotation and relaxation ( μ + SR ) spectra in the temperature ( T ) range between 1.8 and 20 K. All μ + SR parameters, such as the dipolar interaction frequency ( f μ OH ) due to the formation of μ + –OH, its asymmetry ( A μ OH ) and relaxation rate ( σ μ OH ), are found to be independent of T both above and below T m . This is a clear indication that the transition at T m is not due to magnetic order but has a structural and/or electronic origin. In addition, we have been able to determine the muon stopping site for this misfit layered compound. It is found to be located within the CaOH blocking layers, where the muon and OH-group form a water-like H μ + O unit as indicated by a very slow oscillation in the zero-field relaxation spectrum.