Thermally activated spin fluctuations in stoichiometric LiCoO2clarified by electron paramagnetic resonance and muon-spin rotation and relaxation measurements

Lithium cobalt dioxide (LiCoO 2 ) belongs to a family of layered CoO 2 -based materials and has considerable interests in both fundamental physics and technological applications in lithium-ion batteries. We report the results of structural, electrochemical, magnetic susceptibility (χ), electron paramagnetic resonance (EPR), and muon-spin rotation and relaxation (μSR) measurements on powder Li x 0 CoO 2 samples, where the nominal Li/Co ratios (x 0 ) were 0.95, 1.00, 1.02, 1.05, and 1.10, respectively. Structural, electrochemical, and χ measurements suggested that the sample with x 0 = 1.02 is very close to single stoichiometric LiCoO 2 (ST-LCO) phase and that the Co ions in the x 0 = 1.02 sample are in a nonmagnetic low-spin state with S = 0 (t 6 2g ). However, both EPR and μSR revealed that the x 0 = 1.02 (ST-LCO) sample includes a large amount of nonordered magnetic phase in the temperature (T ) range between 100 and 500 K. The volume fraction of such magnetic phase was found to be ∼45 vol% at 300 K by μSR, indicating an intrinsic bulk feature for ST-LCO. In fact, structural and photoelectron spectroscopic analyses clearly excluded the possibility that the nonordered magnetism is caused by impurities, defects, or surfaces. Because EPR and μSR sense static and dynamic nature of local magnetic environments, we concluded that Co spins in ST-LCO are fluctuating in the EPR and μSR time-windows. We also proposed possible origins of such nonordered magnetism, that is, a spin-state transition and charge disproportionation.