  The temperature dependence of AC susceptibility (ACS) has been measured for a
very high-quality plate-like slightly overdoped YBCO single crystal for
different frequencies and AC magnetic field amplitudes. Frequency dependence of
the ACS is weak irrespective of the magnetic field orientation but significant
effects of field orientation with respect to the CuO2 planes and field
magnitude on real and imaginary components of fundamental ACS were observed.
The height of the loss peak saturates as full penetration of magnetic field is
achieved. The peak temperature, Tp, in \c{hi}" shifts to lower temperatures
with increasing magnetic field amplitude for both HIIc and HIIab. The value of
Tp depends on the orientation of the magnetic field with respect to the
crystallographic axes, illustrating the anisotropy in the magnetic flux
dynamics. The superconducting transition width increases weakly with increasing
magnetic field. The Cole-Cole plot [\c{hi}"(\c{hi}')] shows qualitatively and
quantitatively identical features for HIIc and HIIab, independent of the
orientation of the magnetic field with respect to the sample geometry and
shielding current paths. The general features of \c{hi}"(\c{hi}') implies that,
there is no flux creep for the range of frequencies and AC fields employed in
this investigation. The maximum value of the loss peak and its position with
respect to \c{hi}' in the Cole-Cole plot are largely consistent with the Bean
critical state model. Slightly increased peak value in comparison to the
predicted peak value within the Bean critical state model is probably due to a
weak field dependence of Jc. The results obtained here are compared with
various theoretical models and experimental findings. Prominent differences are
noted and discussed in details in this study.
