7.2 Experimental Characterization

Precipitation hardened Sm(Co,Fe,Cu,Zr)$_{7.5-8}$ magnets are classified as ``pinning controlled'' [81,82] and their behavior arises from the cellular precipitation structure, which is observed in transmission electron micrographs. Figs. 7.1, 7.2 show the microstructure of typical Sm(Co,Fe,Cu,Zr)$_{7.5-8}$ type magnets. The magnetic properties are determined by the fine cell morphology with rhomboidal cells of Sm$_2$(Co,Fe)$_{17}$ with a typical diameter of 100-200 nm, which are separated by a boundary phase of Sm(Co,Cu,Zr)$_{5-7}$ [83]. The cellular precipitation structure is formed during a lengthy production process with sophisticated heat treatment, which includes sintering, homogenizing, quenching, isothermal aging, and annealing [12]. Its development is determined by the direction of zero deformation strains due to the lattice misfit between the different phases [84]. The quality of this intercellular phase strongly depends on the additives, especially Zr and Cu. Cu is concentrated in the intercellular ``1:5'' phase, whereas Zr is mainly found in the lamellar structure of the Z-phase and provides a diffusion path for Cu segregation.

Foucault images of Lorentz electron microscopes show that the cellular precipitation structure acts as a pinning site for magnetic domain walls (Fig. 7.3) [85,86,87,12]. The difference in composition between the cells and the cell boundary phase gives rise to a difference in the magnetocrystalline anisotropy. As a result it is energetically favorable for a magnetic domain wall either to stay in the cell boundary phase (``attractive domain wall pinning'' if the domain wall energy is lower) or just inside the cells (``repulsive domain wall pinning'' if the domain wall energy in the cell boundary phase is higher than that in the cells) [88].

Figure 7.1: TEM micrograph of the microstructure of a Sm(Co,Fe,Cu,Zr)$_z$ precipitation hardened magnet. The lamella phase perpendicular to the $[0001]$ direction of the cell matrix phase gives strong contrast in this image [89].

Figure 7.2: Bright field TEM micrograph of the cellular precipitation structure of a Sm(Co,Fe,Cu,Zr)$_z$ magnet. The Sm$_2$(Co,Fe)$_{17}$ cells are surrounded by Sm(Co,Cu)$_5$ cell boundaries [89].

Figure 7.3: Lorentz electron micrograph of a Sm(Co,Fe,Cu,Zr)$_z$ magnet. The magnetic domain wall between two domains with opposite magnetization (bright and dark cells) is pinned on the precipitation structure [87].