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.