Memory effects in an interacting magnetic nanoparticle system.

We have performed a series of measurements to study the low temperature dynamics of an interacting magnetic nanoparticle system. The results obtained demonstrate striking memory effects in the dc magnetization and magnetic relaxation that support the existence of a spin-glass-like phase in interacting magnetic nanoparticles. Moreover, we observe an asymmetric response with respect to temperature change that supports a hierarchical picture, rather than the droplet model discussed in other works on nanoparticle systems.