8.2 Finite Element Model

A HREM image and our geometrical model are shown in Fig. 8.2. We have split our hexahedral particle into six parts of equal volume. In each part the magnetocrystalline anisotropy axis is uniform, but we have varied the axes in the different parts.

Figure 8.2: Left: HREM image of a typical ordered FePt nanoparticle with three regions of different easy axes (picture from [101]).
Right: Geometrical model of an FePt nanoparticle used in the micromagnetic simulations. The finite element mesh consists of 10340 nodes and 55076 elements, which gives a discretization length of 1.2 nm, if the edge length is 30 nm. The model is split into six parts of equal volume, in which the anisotropy axis are varied (2:2:2 configuration shown).

Typical values for the material parameters of FePt (L1$_0$) thin films and nanoparticles have been measured and published in various papers. They are summarized in the papers by Klemmer [102] and Weller [99]. We have chosen the values given in Tab. 8.1 for our FE simulations. Additional influences from surface anisotropy, thermal effects, or disorder in the crystalline structure (dislocations, twins) have been neglected.

Table 8.1: Material parameters of FePt (L1$_0$), which have been used for the micromagnetic simulations [102,99].

FePt (L1$_0$)
$J_\mathrm{s}$	$1.43$ T
$A_\mathrm{exch}$	$1.0\times 10^{-11}$ J/m
$K_\mathrm{ani}$	$7.7$ MJ/m$^3$
$H_\mathrm{ani}$	$10769$ kA/m ($\hat= 13.5$ T)
$l_\mathrm{exch}$	$1.2$ nm