Surface effects on the magnetic behavior of nanoparticle assemblies

We study the interplay between intrinsic properties and collective effects in assemblies of magnetic nanoparticles. Analytical expressions for the magnetization are obtained for weak dipolar interactions in dilute assemblies. Our study is based on thermodynamic perturbation theory for the effective macrospin model where a nanoparticle is represented by its macroscopic magnetic moment, taking into account surface effects of each nanoparticle. The approximate analytical expressions for the magnetization are compared to Monte Carlo simulations and their range of validity is established. Our calculations show that the magnetization is influenced by the nanoparticle anisotropy and the shape of the assembly and that in all cases the effect of the second-order dipolar interaction term of the perturbation theory is the reduction of the magnetization.