Annihilation of antiferromagnetic order in LiCoO2 by excess Li

In order to elucidate the origin of antiferromagnetic (AF) order below 30 K in LiCoO2, in which all the Co3+ ions are in a low-spin state with S=0, the magnetic nature of the Li-excess sample Li1.04Co0.96O1.96 was studied by muon-spin spectroscopy in the temperature range between 1.8 and 100 K. Although disordered localized moments appeared below 25 K, static AF order was not detected even at 1.8 K. Moreover, a small amount of excess Li ions (4%) and oxygen vacancies (2%) was found to change ∼50% of the sample into a magnetically disordered phase at 1.8 K. The stoichiometric LiCoO2, which was prepared from the same starting materials to those for the Li-excess sample, showed an AF transition at 30 K, while the volume fraction of the AF phase was 10% even at 1.8 K. This therefore excludes the possible role of the excess Li+ on the formation of static AF order.