Micro- and macroscopic magnetism on layered cobalt dioxide LixCoO2 (0.1≤ x ≤ 1)

The magnetism for the electrochemically prepared Li,CoO2 samples with 0.1 <= x <= 1 was investigated by muon-spin rotation/relaxation (mu SR) and susceptibility (chi) measurements. The chi(T) curves with H = 10kOe, which correspond to the change in macroscopic magnetism, are almost T-independent down to similar to 50K for the x <= 0.44 samples, suggesting that the samples in this x range are Pauli-paramagnetic. For the samples with 0.49 <= x <= 3/4, a cusp is, however, observed around 170K in their chi(T) curve. Since there are no anomalies in the mu SR parameters around 170 K and VSR is sensitive to local magnetic nature, the cusp is most unlikely due to a magnetic transition, but either a spin state transition or a structural phase transition. Furthermore, zero-field tSR time spectra for all the samples mainly show a slow Kubo-Toyabe behavior, indicating that muon spin is mainly depolarized by randomly oriented nuclear moments of Li-7 and Co-59. Hence, it was clarified that all the LixCoO2 samples with x < 1 are paramagnetic even at 1.8 K, in contrast to NaxCoO2 and KxCoO2. (c) 2007 Elsevier Ltd. All rights reserved.