  The DC and AC magnetic susceptibilities of Na0.85CoO2 single crystals were
measured for the different crystal orientations of H//(ab)- and H//(c)-axis.
The DC-magnetic susceptibility for H//(c)-direction exhibited the
antiferromagnetic transition at TN= 22 K. The thermal hysteresis between the
zero-field-cooled (ZFC) and the field-cooled (FC) magnetization below TN and
the large frustration parameter indicated the spin frustration along the
(c)-axis. For an applied magnetic field in H//(ab)-plane, the DC magnetic
susceptibility exhibited the logarithmic divergent behavior at low temperatures
(T < 6.8 K). This could be understood by the impurity spin effect, dressed by
the spin fluctuation. From the AC magnetic susceptibility measurements, the
real part of the AC-susceptibility for H//(ab) exhibited the spin glass-like
behavior at low temperatures (T < 4 K). Remarkably, for an applied AC magnetic
field with H//(c)-axis, the sign of the AC magnetic susceptibility changed from
a positive to a negative value with increasing AC magnetic field frequency (f >
3 kHz) at low temperatures (T > 7 K). We interpret the sign change of AC
magnetic susceptibility along the (c)-axis in terms of the sudden sign reversal
of the phase difference from in-phase to out-of-phase response with an applied
AC magnetic field in the AC-susceptibility phase space.
