Modelling the Influence of the Magnetic Dipolar Interactions in the Behavior of Nanoparticle Systems

KELIN TAPIA VILLARROEL; BAB, MARISA ALEJANDRA; G P SARACCO

Buenos Aires

Congreso; StatPhys 27; 2019

IUPAP-UBA

Recently, the study of magnetic nanoparticles has received considerable attention, due to their potential applications, especially in the health area, for example, controlled drug deliverance and magnetic hyperthermia. The magnetic behavior of a single-domain system is influenced by their magnetic anisotropies and Zeeman effect. The former is responsible for the ?blocking? phenomena characterized by a blocking temperature where the system changes from the superparamagnetic (high temperature) state to a blocked one (low temperature). The case of nanoparticles with uniaxial anisotropy that are disabled to rotate have been modelled via the Stoner-Wohlfarth model with thermal fluctuations. In this model, the energy of each nanoparticle includes only the interaction between the magnetic moment with both its anisotropy axis and the external magnetic field. However, in real systems long-range magnetic dipolar interactions play a crucial role. We address the question of their influence on the coercive field and the blocking temperature, employing Monte Carlo simulations with Metropolis transition rates and solid angle restriction. The work is divided in two parts. In the first part, iron nanoparticles with diameter D=7.5nm are disposed in random positions inside a box of side L=nD ( n=100, 50, 25, 18, and 12). By performing hysteresis loops, the coercive field, saturation magnetization and blocking temperature are obtained as function of the particle density, temperature, and the angle between the anisotropy axis and the external field. Our findings verify that the coercive field decreases with temperature and angle, but increases with density at high temperatures. So, the blocking temperature increases with density and consequently with the strength of the dipolar interactions. In the second part, nanoclusters of crystallographically aligned nanoparticles of iron-oxide magnetite are modelled using compact nanoclusters formed by 14 nanoparticles of 8nm diameter and same anisotropy. A low-density of nanoclusters is considered in order to discard the intercluster dipolar interactions with respect to the intracluster ones. The results show that, the cluster evolves in such a way that the particle moments predominantly align with the anisotropy axis direction. The magnetization component in the anisotropy axis direction increases when the temperature is reduced, as expected in ferromagnetic arrangements.