Uma investigação sobre a existência de desigualdades fundamentais em processos magnéticos no regime linear

Abstract

In this thesis we investigate the existence of fundamental limits and inequalities in magnetic processes in which nanoparticle systems are under the action of low-amplitude magnetic fields, that is, in processes in which the linear regime of magnetization proves to be valid. First, considering magnetic processes in thermodynamic equilibrium, we find inequalities associated to the equilibrium magnetic susceptibility, saturation magnetization and effective anisotropy constant in blocked magnetic nanoparticles systems. By means of an experimental evaluation, using a sample of barium hexaferrite nanoparticles (BaFe12O19), we verify that such inequalities, in fact, are not violated. Still in the context of equilibrium processes, we show that the inequality found for the susceptibility in blocked nanoparticles must also be valid for non-interacting superparamagnetic systems. Through the evaluation of the experimental results obtained in superparamagnetic systems of magnetite (Fe3O4), magnesium ferrite (MgFe2O4) and zinc ferrite (MgFe2O4), we verify that this inequality is violated in all cases, a fact that we believe to be a signature of the existence of correlations mediated by exchange forces between nanoparticles within the systems. Next, considering dynamic processes, we also obtain inequalities associated to the real and imaginary components of the dynamic susceptibility in addition to the power limit that can be generated through the interaction of alternating magnetic fields with superparamagnetic nanoparticles. In order to evaluate the validity of our theoretical approach also for magnetodynamic processes, we carry out calorimetric procedures under alternating magnetic field effect using the same superparamagnetic samples considered in the equilibrium processes and verify the total agreement of the experimental results with our theoretical expectations. In summary, in this thesis we unequivocally demonstrate the existence of fundamental inequalities that limit the behavior of magnetically blocked and superparamagnetic non-interacting nanoparticle systems in addition to the implications of their violations. Such results are promising tools for maximizing the efficiency of magnetic nanoparticles in biotechnological and biomedical applications since they elucidate the real impacts of effects that are still poorly understood in the literature, such as the effect of inter-particle interactions in magnetic processes.