  The dependence of thermally induced spontaneous magnetization reversal on
time-dependent cooling protocols was studied. Slower cooling and longer waiting
close to the N\`{e}el temperature of the antiferromagnet ($T_N$) enhances the
magnetization reversal. Cycling the temperature around $T_N$ leads to a thermal
training effect under which the reversal magnitude increases with each cycle.
These results suggest that spontaneous magnetization reversal is energetically
favored, contrary to our present understanding of positive exchange bias.
