We would like to indicate that a unified treatment of the whole magnetic problem and its temporal, length and energy scales is in nowadays a challenge. We have concentrated our efforts in some parts and problems related to the total multiscale scheme.
This thesis presents a theoretical study of the magnetic behavior of the low dimensional systems such as nanoparticles and thin films. Most of the magnetic parameters used in the study correspond to the cobalt fcc ones. We have selected the Co because it is one of the typical and widely used for applications magnetic material.
This thesis presents the way to parameterize multispin particle as an effective macrospin with mixed anisotropy (a combination of an uniaxial and a cubic contributions), which has been done in Chapter 2 and Chapter 3. Since the ab-initio calculations on nanoparticles of reasonable sizes, taking account of all on-site parameters are not feasible at the present moment, we have used a phenomenological Néel surface anisotropy model. The variety of nanoparticles with different shapes, underlying structures, strengths of the surface anisotropy, etc has been investigated. Such approximation will in the future open the doors for modeling of an assemble of nanoparticles as a set of effective macrospins which implicitly takes into account the effects of the surface, underlying lattice structure and shape.
During the work on this thesis the method of the constrained Monte Carlo has been developed in collaboration with Dr. P. Asselin (Seagate Technology, USA) and co-workers. This new method allows to evaluate numerically the dependence on temperature of the magnetic anisotropy in magnetic nanoparticles and thin films. We have used it in the study of the dependence on temperature of the magnetic anisotropy in magnetic thin films and nanoparticles with Néel surface anisotropy, in Chapter 4 and Chapter 5. In those chapters of the thesis we have analyzed the applicability of the new method, have studied the deviations from the Callen-Callen law of the dependence on temperature of macroscopic magnetic anisotropy in the presence of the surface anisotropy. We have also studied the re-orientation transition in thin films with the surface anisotropy. Finally, we also show the possibility to have a temperature-induced transition from cubic (due to surface effects) to uniaxial macroscopic anisotropy in nanoparticles.
In Chapter 6 we use a multiscale scheme, starting from the "ab-initio" calculations and using a fully relativistic screened Khon-Korringa-Rostocker (SKKR) method . The "ab-initio" simulations have been performed with a code supplied by Dr. L. Szunyogh from Budapest University, Hungary. The magnetic properties of Co (bulk), semi-infinite Co ((100) and (111)), and system are studied. The use of this method allows to extract local magnetic parameters and parameterize the Heisenberg Hamiltonian. Inside the multi-scale scheme, the constrained Monte Carlo method is applied to evaluate the temperature-dependent parameters, including the macroscopic anisotropy in the presence of the Ag surface.