7.4 Rhomboidal Cell Structure

A finite element model of the microstructure of Sm(Co,Fe,Cu,Zr)$_z$ has been developed. It consists of $2 \times 2 \times 2$ rhomboidal cells with a spacer layer for the cell boundary phase in between (see Fig. 7.13). The edge length $e$ and the ``corner angle'' $\beta$ of the rhombohedrons as well as the thickness $t$ of the precipitation are variable. The ``space diagonal'' $D$ is parallel to the easy axis. The domain wall of the initial magnetization distribution of our simulations lies in the plane, which is indicated by the thick lines.

Figure 7.13: Finite element model of the rhomboidal cell structure of precipitation hardened Sm(Co,Fe,Cu,Zr)$_z$ magnets.

We have assumed the following material parameters for 300 K [80]: For the cells (``2:17'' type) $J_\mathrm{s}=1.32 \mathrm{T}$, $A=14 \mathrm{pJ/m}$, $K_1=5 \mathrm{MJ/m}^3$. For the cell boundary phase (``1:5'' type) we have used $J_\mathrm{s}=0.8 \mathrm{T}$, $A=14 \mathrm{pJ/m}$, $K_1=1.9 \mathrm{MJ/m}^3$. The resulting exchange length is $1.7 \mathrm{nm}$ in the cells and $2.7 \mathrm{nm}$ in the cell boundary phase. Thus, the domain wall width is $5.3 \mathrm{nm}$ in the cells and $8.5 \mathrm{nm}$ in the cell boundary phase.

These material parameters with lower anisotropy in the intercellular phase as compared to the cells give rise to ``attractive pinning'', which means, that the domain wall prefers to move into the intercellular phase and stays there pinned.