The effects of a dipole-dipole interaction on the magnetic relaxation in a magnetic nanoparticle system have been investigated using ac-susceptibility, magnetic relaxation, and magnetic noise measurements, which altogether cover a time window of nine decades. In the experimental time window, it is possible to distinguish two temperature regimes: a low-temperature regime where collective dynamics is probed, as evidenced by the appearance of magnetic aging and a significantly broadened magnetic relaxation, and a high-temperature regime where the magnetic relaxation is best described as single-particle dynamics. Moreover, the magnetic relaxation of the interacting particle system is shifted towards longer time scales, but gradually, at high temperatures, the relaxation rate approaches the rate of the noninteracting sample.

• Received 1 July 1997

DOI:https://doi.org/10.1103/PhysRevB.57.497