Nonlinear Magnetization Dynamics in Thin-Films and Nanoparticles.
[Tesi di dottorato]
Tesi di dottorato
||Nonlinear Magnetization Dynamics in Thin-Films and Nanoparticles
|Number of Pages:
||Università degli Studi di Napoli Federico II
|Number of Pages:
||micromagnetics, Landau-Lifshitz-Gilbert equation, precessional switching, spin-polarized currents, perturbative tecniques, micromagnetic simulations, fast switching, geometrical integration, micromagnetic standard problem n. 4
||Area 09 - Ingegneria industriale e dell'informazione > ING-INF/02 - Campi elettromagnetici
||02 Mar 2007
||30 Apr 2014 19:22
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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