D'Aquino, Massimiliano (2005) Nonlinear Magnetization Dynamics in Thin-Films and Nanoparticles. [Tesi di dottorato] (Unpublished)

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Item Type: Tesi di dottorato
Language: English
Title: Nonlinear Magnetization Dynamics in Thin-Films and Nanoparticles
Creators:
CreatorsEmail
D'Aquino, MassimilianoUNSPECIFIED
Date: 2005
Date Type: Publication
Number of Pages: 165
Institution: Università degli Studi di Napoli Federico II
Department: Ingegneria elettrica
PHD cycle: 17
PHD Coordinator:
nameemail
Miano, GiovanniUNSPECIFIED
Tutor:
nameemail
Serpico, ClaudioUNSPECIFIED
Date: 2005
Number of Pages: 165
Uncontrolled Keywords: micromagnetics, Landau-Lifshitz-Gilbert equation, precessional switching, spin-polarized currents, perturbative tecniques, micromagnetic simulations, fast switching, geometrical integration, micromagnetic standard problem n. 4
MIUR S.S.D.: Area 09 - Ingegneria industriale e dell'informazione > ING-INF/02 - Campi elettromagnetici
Date Deposited: 02 Mar 2007
Last Modified: 30 Apr 2014 19:22
URI: http://www.fedoa.unina.it/id/eprint/148

Abstract

In chapter 1 the micromagnetic model and the Landau-Lifshitz-Gilbert (LLG) equation is introduced to describe magnetization phenomena in ferromagnetic bodies. First, an approach in terms of the free energy associated with the magnetic body is presented to derive the static equilibrium conditions for magnetization vector field. Then, the dynamic effects due to the gyromagnetic precession are introduced. Both Landau-Lifshitz and Landau-Lifshitz-Gilbert equation are presented. Phenomenological Gilbert damping is analyzed in terms of Rayleigh dissipation function. In chapter 2 the study of magnetization dynamics in uniformly magnetized particles is addressed. In particular, first the static Stoner-Wohlfarth model and then magnetization switching processes are analyzed. In addition, novel analytical techniques to study magnetization dynamics under circularly polarized external fields and magnetization dynamics driven by spin-polarized currents are introduced and deeply discussed. In this respect, it is shown how some behaviors indeed observed in experiments, can be explained in terms of bifurcations of fixed points and limit cycles of the LLG dynamical system. As a further step, in chapter 3, the assumption of magnetization spatial uniformity is removed and the problem of studying thin-films reversal processes of technological interest is addressed. In this respect, as preliminary step, the issue of the computation of magnetostatic fields, which is still the bottleneck of micromagnetic simulations, is illustrated together with the mostly used methods at this time. Then, a comparison of damping and precessional switching processes in thin-films is performed, showing that fast precessional switching can be considered spatially quasi-uniform and, therefore, its crucial aspects can be analyzed by means of uniform mode theory discussed in chapter 2. Finally, a uniform mode analysis is applied to the fast switching of granular tilted media which represents one of the most promising solutions for high density magnetic storage in future hard disks. In chapter 4, the problem of the geometrical integration of LLG equation is considered. In particular, the mid-point rule time-stepping is applied to the LLG equation. In fact, it is shown that the fundamental properties of magnetization dynamics, embedded in the continuous model, are reproduced by the mid-point discretized LLG equation regardless of the time step. In addition, since the resulting numerical scheme is implicit, special and reasonably fast quasi-Newton technique is developed to solve the nonlinear system of equations arising at each time step. The proposed mid-point technique is validated on the micromagnetic standard problem no. 4 which concerns with thin-films reversal processes. Finally, numerical results and computational cost are discussed.

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