Thermally activated spin fluctuations in stoichiometric LiCoO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow/><mml:mn>2</mml:mn></mml:msub></mml:math>clarified by electron paramagnetic resonance and muon-spin rotation and relaxation measurements

Abstract

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 (\ensuremath{\chi}), electron paramagnetic resonance (EPR), and muon-spin rotation and relaxation (\ensuremath{\mu}SR) 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 \ensuremath{\chi} 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 (${t}_{2g}^{6}$). However, both EPR and \ensuremath{\mu}SR 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 \ensuremath{\sim}45 vol% at 300 K by \ensuremath{\mu}SR, 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 \ensuremath{\mu}SR sense static and dynamic nature of local magnetic environments, we concluded that Co spins in ST-LCO are fluctuating in the EPR and \ensuremath{\mu}SR time-windows. We also proposed possible origins of such nonordered magnetism, that is, a spin-state transition and charge disproportionation.