  We have performed a series of measurements on the low temperature behavior of
a magnetic nano-particle system. Our results show striking memory effects in
the dc magnetization. Dipolar interactions among the nano-particles {\em
suppress} the memory effect. We explain this phenomenon by the superposition of
different super paramagnetic relaxation times of single domain magnetic nano-
particles. Moreover, we observe a crossover in the temperature dependence of
coercivity. We show that a dilute dispersion of particles with a flat size
distribution yields the best memory.
