Anisotropic magnetization and sign change of dynamic susceptibility in   Na0.85CoO2 single crystal
  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. 
J. S. Rhyee, J. B. Peng, C. T. Lin and S. M. Lee