Dynamics of interacting magnetic nanoparticles: effective behavior from competition between Brownian and Néel relaxation.

The intriguing properties of magnetic nanoparticles have sparked a growing number of theoretical studies as well as practical applications. Here, we provide the first comprehensive study of the influence of interactions on the two main relaxation mechanisms: internal (Néel) and Brownian relaxation. While non-interacting magnetic nanoparticles show Debye behavior with an effective relaxation time, many authors use this model also for the interacting case. Since Néel relaxation is typically a thermally activated process on times scales that are many orders of magnitude larger than the underlying micromagnetic times, we use extensive computer simulations employing a Brownian dynamics/Monte-Carlo algorithm to show that dipolar interactions lead to significant deviations from the Debye behavior. We find that Néel and Brownian relaxation can be considered as independent processes for short enough times until dipolar interactions lead to a coupling of these mechanisms, making the interpretation more difficult. We provide mean-field arguments that describe these short and long-time, effective relaxation times well for weak up to moderate interaction strengths. Our findings about the coupling of Brownian and Néel process and the effective relaxation time provide an important theoretical insight that will have also important consequences for the interpretation of magnetic susceptibility measurements and magnetorelaxometry analysis.