Crystalline and magnetic microstructures of iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 sintered magnets: Isothermal aging effect

Abstract

Crystalline and magnetic microstructures of the iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 anisotropic sintered magnets after isothermal aging process were investigated systematically by using transmission electron microscope (TEM) and magnetic force microscopy (MFM). As the isothermal aging time (t) increases from 0.5 to 20 h, the remanences of the specimens almost keep constant (11.4 kGs), while the intrinsic coercivity (Hcj) shows an increase from 1.13 to 7.93 kOe, which is much higher than the well-known value of the order of 1 kOe. TEM investigation indicates that the cellular microstructures are incomplete together with partially phase transformation of 1:7H to 2:7R and 5:19H when t = 0.5 h. When t reaches 5 h, the phase transformation is almost completed. With t increasing from 2.5 to 20 h the average cellular size of the specimens increases from ∼90 to ∼150 nm and the density of the lamellae phase increases from ∼0.014 to ∼0.033 1/nm. MFM investigations show that the average surface width (Ws) decreases from 0.86 to 0.67 μm initially and then increases to 0.98 μm and the main domain width (Wm) decreases from 2.17 to 1.21 μm initially and then increases to 1.51 μm. And the Wm is about two times of Ws for the specimens because of the smaller magnetostatic energy of the surface layer of the specimens with c-axis parallel to the imaging plane. It is impressive that the domain walls form in straight line through cellular structures as t is less than 5 h, and in a zig-zag shape when t is over 5 h. This is the first time to reveal systematically the domain structures for the iron-rich 2:17-type Sm-Co magnets during isothermal aging process. The evolution of the magnetic microstructures is ascribed to the variation of the effective intrinsic magnetic properties of cell and cell boundary phases originating from both the phase transformation and development of cellular structures.