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.
@article{arxiv.cond-mat/0407202,
title = {Polydispersity-linked Memory Effects in a Magnetic Nanoparticle System},
author = {1. S. Chakraverty and 2. A. Frydman and 3. M. Bandyopadhyay and 4. S. Dattagupta and 5. Surajit Sengupta 6. P. A. Sreeram},
journal= {arXiv preprint arXiv:cond-mat/0407202},
year = {2007}
}