  We have found a sharp transition in the vortex creep rates at a temperature
T^\ast=0.05 T_c in a single crystal of Sr_2RuO_4 (T_{c}=1.03 K) by means of
magnetic relaxation measurements. For T<T^\ast, the initial creep rates drop to
undetectable low levels. One explanation for this transition into a phase with
such extremely low vortex creep is that the low-temperature phase of Sr_2RuO_4
breaks time reversal symmetry. In that case, degenerate domain walls separating
discreetly degenerate states of a superconductor can act as very strong pinning
centers.
