Theoretical studies of magnetic materials at the micrometer and nanometer scale

Abstract

In this thesis we analyzed, from a theoretical point of view, different aspects of the magnetic system at the micrometer and nanometer scale. Firstly, we studied the applications of the magnetostatic of continuous media theory in two different magnetic materials of cylindrical shape. In particular, we analyze the effect of magnetoelastic phenomena in microwires and found a temperature dependence of the switching fleid which well compares with the experimental data. Also, we calculated the magnetostatic interaction energy in an array of magnetic nanowires considering the magnetic structure of each nanowire as a single ferromagnetic domain, we show that the dipole—dipole interaction among the wires overestimates the true interaction energy by several orders of magnitude at closer interwire distances. Secondly, we analyze the dynamics of two interacting nanoparticles in the presence of an applied magnetic field including the dipolar and exchange interactions at a fixed temperature. In the case of nuli temperature, the system has a deterministic behavior and we found that depending on the interaction strengths, two time scales appear: a longer one associated with the predominance of the exchange interaction and a shorter time scale associated with a greater dipolar interaction. In addition, we found that the magnitude of the total system magnetization is not constant; but it is a fluctuating time dependent function. When the thermal noise effect is taken into account, a disorder in the magnitude of total magnetization of the system is produced, the disorder increases when the dipolar strength decreases. Finaily, we studied the equilibrium statistical mechanics for noninteracting anisotropic nanoparticles. The thermodynamics quantities, in the context of the canonical ensemble, are derived using our analytical expression for the partition function and we found that, a maximum of the magnetization curve as funetion temperature is attained when the anisotropy field is perpendicular to the external magnetic field and that the inverse of the susceptibility curve as function of temperature exhibit different behaviors depending on the anisotropy field, which are as those typical of interacting particles.