  We present ac susceptibility measurements with the frequency spanning three
orders of magnitude on single grain, icosahedral R - Mg - Zn (R = rare earth)
quasicrystals. The freezing temperature in Gd-based, Heisenberg spin glasses in
this family increases by ~ 2% with a frequency increase from 10 Hz to 10 kHz,
whereas the freezing temperature in the non-Heisenberg members of the family is
significantly more responsive to the frequency change (by 16 - 22 %),
suggesting that an additional magnetic anisotropy distribution in the
non-Heisenberg spin glasses causes changes in the low frequency magnetic
dynamics.
