*3.4. Magnetic Properties*

The magnetic characteristics of the Co-4.54%Sn alloy were measured at different undercoolings, as demonstrated in Figure 6. The Co-4.54%Sn alloy shows typical soft magnetic features (Figure 6a). Figure 6b describes the process of domain growth and magnetization rotation under an external magnetic field. The magnetic domains parallel to the applied magnetic field will grow, and the remaining magnetic domains will shrink. With the increase of magnetic field *H*, *B* increases rapidly, and magnetic domain growth takes the way of magnetic domain wall movement. When the external magnetic field is increased, the magnetic rotation begins, and the slope of the curve of *B* relative to *H* becomes smaller. Finally, the magnetic moment direction of each magnetic domain remains horizontal with the direction of the external magnetic field, and the *B* value does not change.

**Figure 6.** Magnetic characteristics of Co-4.54%Sn alloy solidified at different undercoolings. (**a**) Hysteresis loop, (**b**) magnetic domain growth and rotation process, (**c**) Ms and Mr and (**d**) coercivity Hc.

The saturation magnetization is generally related to the composition, the magnetic domain wall and the lattice constant of the material. The magnetic properties of the material are determined by the exchange interaction between the electrons in the material [25–27]. Its coercivity and saturation magnetization mainly depend on the material composition, crystal defect, internal stress and grain size [28]. Table 1 specifies the magnetic parameters of the Co-4.54%Sn alloys at different undercoolings.


**Table 1.** The magnetic property parameters of Co-4.54%Sn alloy under different undercoolings.

With the increase of undercooling, the saturation magnetization of the alloy decreases from 178.92 emu/g to 149.94 emu/g, which is caused by the refinement of the microstructure, and the enhancement of dendritic growth velocity and solute content for the primary phase. Han et al. showed that alloy variants with larger average particle size usually have higher saturation magnetization, which is consistent with this result [29]. Compared with the undercooling of 11 K (3.04 emu/g, 7.3 kA/m), the undercooling of 189 K has lower remanent magnetization and higher coercivity, which are 2.01 emu/g and 12.42 kA/m, respectively. In addition, with the increase of undercooling, the squareness ratio showed an "increase first and then decrease" trend. The remanent magnetization and coercivity also exhibited a trend of increasing first, then decreasing with the increase of undercooling and solute content, as shown in Figure 6c,d. The reason is that the crystal particle and boundary are raised significantly due to the refinement of microstructure and enhancement of solute

content with the increase of undercooling, and a large amount of solute is distributed into the primary phase, which leads to the decrease of saturation magnetization and the increase of the remanent magnetization and coercivity. When the undercooling Δ*T* was 189 K, the remanent magnetization and coercivity were decreased in that the volume fraction of eutectic cell was higher than the undercooling of 11 K. Furthermore, a more in-depth study of the magnetic mechanism is still needed.
