3.2 Comparison between Damping and Precessional switching in magnetic
thin-films

We have seen in section 2.4.2 that traditionally,
magnetization reversal in thin films is realized by applying a
sufficiently large magnetic field almost antiparallel to the
initial magnetization state and that the resulting reversal
dynamics is driven by dissipative processes. This kind of
switching is referred to as damping switching in
literature [26,27].

Figure 3.2:
Sketch of
the thin-film geometry.

Nevertheless, as seen in section 2.4.3, the
possibility of using precessional motion of magnetization to
realize the switching of thin films and particles has been
recently investigated [34,37,68]. In this kind
of switching, referred to as precessional
switching [28], the in-plane external field is
approximately orthogonal to the initial magnetization state and
produces a torque that drives precessional motion of
magnetization; this results in a faster and less energy-consuming
magnetization dynamics. Magnetization reversal is realized by
switching the external field off precisely when precession has
brought the magnetization state close to its reversed orientation.
Therefore, the applied field pulse duration has to be carefully
chosen, while in damping switching there is no such need. Although
it is generally desired that thin films and nanoelements in
magnetic storage devices are in almost uniform magnetization
states, both conventional switching and precessional switching are
nonuniform dynamic processes. Here, we investigate the switching
process of a permalloy magnetic rectangular thin-film: the
thickness is nm, the major and mean edge length are
respectively nm and nm (see
Fig. 3.2). The thin-film medium has a uniaxial
magneto-crystalline anisotropy whose easy axis is along the x-axis
(long axis), the uniaxial anisotropy constant is
J/m, the exchange stiffness constant is
J/m, the saturation magnetization
kA/m (such that
T) and the
damping constant is
; the exchange length of the
material, defined by Eq. 1.92, is

ex nm

(3.19)

We assume that magnetization dynamics of the thin-film is
described by the Landau-Lifshitz-Gilbert
equation (1.83), namely:

(3.20)

where
is the effective field defined by
Eq. (1.68)

which includes the applied field
, the exchange field
exc, the anisotropy field
an and the
magnetostatic (demagnetizing) field
, as seen in
section 1.2.2.
In micromagnetic simulations, the numerical time integration of
Eq. (3.20) is performed by using a backward
differentiation formula [69]; the spatial discretization is
done using the finite element method [52] with a mesh
consisted of tetrahedrons; the mesh is finer near the corners of
the thin-film (mesh edge length
nmexc) where
a stronger accuracy is required for the computation of
magnetostatic field. The hybrid finite element boundary element
method [70], discussed in section 3.1.2, is
used to solve the magnetostatic problem.
All the numerical simulations that we will present in this section
have been performed with the parallel code
MAGPAR [73] developed by W. Scholz at Vienna
University of Technology [80].
First, we perform micromagnetic simulations of conventional
(damping) and precessional switching process for the thin-film.
Initially, the thin-film is saturated along the direction and
then relaxed, by switching the external field off, to the remanent
C-state (see Fig. 3.4, on the right), which is one
of the equilibrium configurations really observed in experiments
on magnetic thin-films^{3.2}. At time the external
field is applied, respectively antiparallel and orthogonal to the
easy axis, as sketched in figure 3.3. We
compare two aspects of the switching processes: the switching
speed and the uniformity of the magnetization during the reversal
process.

Figure 3.4:
Numerical results. Remanent states of magnetic thin-film.
(left) S-state. (right) C-state.