  The peculiar field-dependent magnetism of Na0.82CoO2 has been investigated
through an analysis of its DC and AC spin susceptibilities. To account for the
easily activated narrow b2g-a1g gap of the crystal field for Co in the cobalt
oxide layer, the spin-state transition of Co3+ (3d6) between the low spin (LS)
state b2g^2-a1g^0 of S=0 and the intermediate spin (IS) state b2g^1-a1g^1 of
S=1 is thus seen as thermally activated and exhibits a Boltzmann distribution.
The IS state of Co3+ within each sqrt(13a) hexagonal superlattice formed by the
S=1/2 state of the Co4+ ions appears randomly within each supercell and shows
significant temperature and field dependence. The magnetic field is found to
assist in pinning down the thermally activated state of Co3+ and swings the
Boltzmann distribution weight toward a higher fraction of the IS state. The
field dependence of the in-plane magnetic moment from the added number of S=1
spins is used to explain the origin of A-type antiferromagnetic (AF) ordering,
particularly that the ferromagnetic (FM)-like behavior below TN at low field is
actually a ferrimagnetic IS spin ordering of Co3+.
