Domain structures of two Sm-Co-Cu-Fe-Zr ‘‘2 : 17’’ magnets during magnetization reversal

Domain patterns during magnetization reversal were studied, using the Kerr magneto-optic effect, on sintered magnets of SmCo5Cu0.75Fe1.6Zr0.1 (MHc=6.5 kOe, (BH)m=28.8 MGOe) and SmCo5.36Cu0.64Fe1.76Zr0.24 (24.3 kOe, 23.6 MGOe). In the low-Hc magnet thin sheet reverse domains containing the [00.1] direction originate at grain boundaries. They grow in irreversible spurts along the edges, multiplying and branching, thus gradually filling all grains with dense, wavy patterns. Easy nucleation occurs on a grain boundary phase, while stronger uniform wall pinning by the dense 1–5 precipitate phase inside the 2 : 17 grains controls the coercivity. In the high-Hc magnet, similar grain boundary nucleation is seen in low fields. Additional nucleation inside grains may occur at higher H. Further growth shows ‘‘cloud-like’’ domains expanding fairly isotropically in all directions. Near the coercive field, local full reversal occurs in some large grains, while others remain completely unaffected. Pinning inside grains controls the magnetization reversal, but its strength is quite different for the two 2 : 17 major phase regions.