  The AC susceptibility at zero DC magnetic field of a polycrystalline sample
of LaFeAsO_{0.94}F_{0.06} (T_c = 24 K) has been investigated as a function of
the temperature, the amplitude of the AC magnetic field (in the range Hac =
0.003 - 4 Oe) and the frequency (in the range f = 10 kHz - 100 kHz). The
temperature dependence of the AC susceptibility exhibits the typical two-step
transition arising from the combined response of superconduncting grains and
intergranular weak-coupled medium. The intergranular part of the susceptibility
strongly depends on both the amplitude and the frequency of the AC driving
field, from few Kelvin below T_c down to T = 4.2 K. Our results show that, in
the investigated sample, the intergrain critical current is not determined by
pinning of Josephson vortices but by Josephson critical current across
neighboring grains.
