Frustrated magnetism in the two-dimensional triangular lattice of LixCoO2

Magnetism of the simplest layered cobaltite LixCoO2 with x=1.0, 0.95, and 0.7 has been investigated by positive muon spin rotation and relaxation (µ+SR) spectroscopy together with magnetic susceptibility measurements, using polycrystalline samples in the temperature range between 300 and 1.8 K. Weak transverse field µ+SR showed the appearance of magnetism below ~65 K (=Tcun) for the sample with x=1.0. The volume fraction of the magnetic phase at 1.8 K is determined to be ~20% for LiCoO2, strongly suggesting that the observed magnetism is not induced by impurities but is an intrinsic behavior. This indicates that either a charge disproportionation (2Co3+->Co2++Co4+), a spin state transition (t6/2g->t5/2ge1g), or an appearance of magnetic Co3+ at surface occurs below 65 K. Zero field µ+SR spectra of LiCoO2 consist mainly of a slow relaxing signal due to random fields even at 1.8 K plus a small amount of a fast relaxing signal and an oscillating component, implying the presence of an antiferromagnetic order, as in the case of NaxCoO2 with x0.75. For Li-deficient LixCoO2 samples, T decreases with decreasing x from 50 K for x=0.95 to 25 K for x=0.7. The volume fraction of the magnetic phase is however almost independent of x and is estimated as 20% at 1.8 K, but no oscillations were observed in the zero field spectrum.