Thermomagnetic properties at the nanoscale

Abstract

This Thesis deals with the magnetic properties and magnetocaloric effect (MCE) in the single-domain range. The motivation to carry out such work is based on the unusual magnetic properties that common materials exhibit in nanoscaled dimensions when they reach the single-domain size, often radically different and/or enhanced with respect to their bulk counterparts. These new properties have a wide range of technological applications, ranging from magnetic recording to biomedicine. In particular, the study of the MCE in these low-dimensional systems is of primordial importance both for refrigeration purposes of micro- and nano-electro mechanical systems, and for biomedical applications as magnetic agents for hyperthermia treatments. Characterizing the magnetic properties (and the MCE) in these reduced dimensions is very complex, since the magnetic response of the system is strongly dependent in several factors as size, shape, anisotropy, dipole-dipole interaction, etc, which make difficult to control the parameters ruling its behaviour, and consequently, limit their technological use. Furthermore, single-domain magnetic systems may exhibit superparamagnetic (SPM) behaviour depending on the specific conditions (applied magnetic field, temperature, magnetic anisotropy, size, shape, etc). SPM behaviour is the paramagnetic-like temperature dependence that single domain magnetic entities may exhibit at certain conditions, and it is evident that needs to be perfectly controlled depending on the specific applications we are interested in (for example, for magnetic recording purposes it is necessary to avoid SPM fluctuations, so that the magnetic information remains stable against thermal fluctuations). In this context, the use of a computational technique (Monte Carlo one in our case) arises as a very useful tool to study such magnetic nanostructures: on the one hand, with a MC method the characteristics of the system are perfectly controlled and, on the other hand, we can study problems with no analytical solution, as for example the magnetic dipole-dipole interaction. This is the main objective of the present work: with the help of a MC technique we can study different nanostructured systems, as randomly distributed nanoparticles systems or chain-like nanoparticle assemblies, and to investigate how the different parameter (magnetic anisotropy, size, shape, interparticle interactions, etc) rule its behaviour. This knowledge will then be applied to search for the optimizing MCE-based applications both for hyperthermia and refrigeration purposes.