Granular metallic systems as interacting superparamagnets: anhysteretic magnetization and hysteresis loops

Abstract

The magnetization properties of granular bimetallic systems are discussed in detail. These systems can never be described as simple superparamagnets, because of the presence of non-negligible dipolar interactions among nanoparticle moments. Their anhysteretic magnetization can be well represented over a wide temperature range by a superposition of Langevin functions whose argument must be properly modified to account for interactions. Granular systems are therefore described as being in a particular regime, called the interacting superparamagnetic regime, and features of these are briefly discussed. Dipolar magnetic interactions are also responsible for the hysteretic behavior of these materials. A recent mean-field model for the hysteresis loops is shown to perfectly describe the temperature behavior of their shape and the hysteretic properties of the simulated magnetic response of an assembly of interacting equal moments.