Combining dipolar and anisotropic contributions to properly describe the magnetic properties of magnetic nanoparticles real systems

Abstract

The magnetic properties of a real system of magnetite nanoparticles with controlled interparticle distances via a silica shell are modeled by the modification of existing theoretical models that describe ideal non-interacting superparamagnetic systems. In this work, the variation of the blocking temperature as a function of the interparticle separation is explained through a phenomenological model where the interaction is taken into account through a dipolar field that modifies the intrinsic anisotropy field of the system. Moreover, it is observed that the field-dependent magnetization of the studied samples does not fulfill the universal scaling law of superparamagnetic systems, in which the magnetization is well described by the classic Langevin model, even for the less interacting samples. However, when the actual temperature of the system is modified by a temperature factor comprised by two terms that account for dipolar and anisotropy contributions, the magnetization curves satisfactorily comply with the scaling law. The results suggest that the interaction increases the anisotropy barrier and the developed approach allows to distinguish the effect of this contribution from the anisotropic contribution on the magnetic properties studied in this system. By means of this study is demonstrated that models like the Interacting Superparamagnetic model must be carefully used to describe correctly a non-interacting system because the latter can accounts for a false interaction that is not present from blocking temperature measurements.