Monte Carlo study of the magnetic properties of frozen and non-interacting nanoparticles

The transition from the blocked to the superparamagnetic regime in non-interacting and frozen magnetite nanoparticles with diameters of about 10 nm was studied using the Monte Carlo method with the Metropolis algorithm. The behavior of the blocking temperature (T _B) was analyzed for different nanoparticle systems. For ensembles of homogeneous nanoparticles, T _B showed a linear dependence on the exchange constant, which is the main factor that determines T _B. Comparatively, the dependence of T _B on the magnetocrystalline anisotropy constant was much weaker and nonlinear. It was observed that T _B decreases with the decreasing particle size following a finite-size scaling theory. Systems of nanoparticles with a core/dead-layer structure exhibited a lower T _B than the corresponding homogeneous nanoparticles. It was verified that the presence of a thin, hard layer on the nanoparticles surface, where the exchange interaction was improved, produced a significant increase in the blocking temperature.