Magnetic nature of KxCoO2 near the antiferromagnetic phase with x=0.5: Positive muon spin rotation and relaxation
Résumé
In order to elucidate the magnetic nature of KxCoO2 in the vicinity of x=0.5, we have measured positive muon spin rotation and relaxation (µ+SR) spectra using polycrystalline samples of x=0.6, 4/7, and 0.5 in the temperature range between 1.8 and 300 K. A zero-field spectrum suggests the existence of localized but disordered moments below 20 K for the x=0.6 sample, whereas the spectra reveal the absence of magnetic moments in the x=4/7 sample. Combining our µ+SR results with the results of resistivity and susceptibility measurements, it is found that the phase boundary between the Curie-Weiss metal and Pauli-paramagnetic metal exists between x=0.6 and 4/7. For the rhombohedral x= 0.5 sample (i.e., β phase), the quasistatic antiferromagnetic (AF) order appears below 58 K (=TN), and the whole sample enters into the AF phase below TN, as in the case for the hexagonal γ-K0.5CoO2. This suggests that the interplane interaction is most unlikely to be crucial for determining the magnetic nature of the CoO2 plane. The complex T dependence of the internal AF fields, particularly the drastic change at the charge-order transition temperature (TCO= 20 K), is qualitatively explained by the change in the muon sites induced by charge order in the CoO2 planes
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