Effect of positional disorder in systems of ultrafine ferromagnetic particles

A simple model for systems of dipolarly interacting single-domain ultrafine ferromagnetic particles is studied by Monte Carlo simulations of zero field cooling and field cooling as well as relaxation e)experiments. By investigating systems characterized by an identical moderate concentration but different types of particle positions' disorder, it is shown that the, positional disorder has a crucial influence on the magnetic behavior of the system. For extreme values of positional disorder, the interplay between spatial disorder and dipolar interaction can even lead to a cooperative freezing at low temperatures.