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.
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