Universal behavior of dense clusters of magnetic nanoparticles
  A detailed numerical simulation of quasistatic hysteresis loops of dense clusters of interacting magnetic nanoparticles is carried out. Both clusters of magnetically soft and magnetically hard nanoparticles are considered. The clusters are characterized by an average particle diameter D, the cluster radius Rc, the particle saturation magnetization Ms, and the uniaxial anisotropy constant K. The number of particles in the cluster varies between Np = 30 - 120. The particle centers are randomly distributed within the cluster, their easy anisotropy axes being randomly oriented. It is shown that a rare assembly of identical random clusters of magnetic nanoparticles can be characterized by two dimensionless parameters: 1) the relative strength of magneto-dipole interaction, K/Ms^2, and the average particle concentration within the cluster, {\eta} = VNp/Vc. Here V is the nanoparticle volume, and Vc is the volume of the cluster, respectively. In the strong interaction limit, Ms*{\eta}/Ha >> 1, where Ha = 2K/Ms is the anisotropy field, the ultimate hysteresis loops of dilute assemblies of clusters have been constructed. In the variables (M/Ms, H/Ms) these hysteresis loops depend only on the particle volume fraction {\eta}. In the weak interaction limit, Ms*{\eta}/Ha << 1, the assembly hysteresis loops in the variables (M/Ms, H/Ha) are close to the standard Stoner-Wohlfarth hysteresis loop. 
N.A. Usov and O.N. Serebryakova
cond-mat.mes-hall