Magnetic domains in rare-earth cobalt permanent magnets

The microstructure of two commercial 2:17 precipitation hardened rare-earth magnets has been examined and compared with the magnetic properties. Transmission electron microscope studies on Sm(Co,Fe,Cu,Hf)7.25, Hicorex 99C show a fine cellular microstructure with 2:17 rhombohedral cells surrounded by coherent 1:5 hexagonal cell boundaries. In Sm(Co,Fe,Cu,Zr)7.22, TDK REC-26 the cells are much coarser and 1:3 phase thin lamellae are superimposed on the cellular structure. X-ray energy dispersive analysis shows a higher Cu content in the cell boundaries. Lorentz electron microscopy shows "wavy" domain walls in both cases indicating that the domain walls are pinned at the cell boundaries. The higher Cu content of the cell boundaries dilutes the magnetic properties of the SmCo5phase and produces a large domain wall energy gradient across the cell boundary leading to the observed high coercivities.