Paramagnetic nature of the layered cobalt dioxide with a double rocksalt-type layer

In order to elucidate the origin of a small magnetic anomaly at Tm=13K in [Ca0.85OH]1.16RS[CoO2], we have measured μ+SR spectra down to 1.8 K using a pressed powder sample synthesized by a high-pressure technique. The ZF-μ+SR spectrum at 1.8 K is well fitted by a damped precessing signal caused by a formation of μ+-OH bond together with an exponential relaxation function due to a disordered magnetic field. The volume fraction of the former signal is approximately 80%, whereas the latter is ∼20%. Furthermore, all the major μ+SR parameters, such as the two asymmetries (AμOH and Ae), the precession frequency due to the μ+-OH bond, and its Gaussian relaxation rate are independent of T both above and below Tm. This is a clear indication that Tm is not magnetic order but structural and/or electronic in origin.