Presentation Information
[P1-35]Magnetization reversal behavior of die-upset Nd-Fe-B magnets with heterogenous microstructure
*HARIM CHOI1,2, Makoto KOBASHI1, Hossein Sepehri-Amin3, Yusuke HIRAYAMA2 (1. Nagoya university (Japan), 2. National Institute of Advanced Industrial Science and Technology (Japan), 3. National Institute for Materials Science (Japan))
Keywords:
Magnetization reversal
The microstructure of Nd-Fe-B magnets produced by the semi-solid process closely resembles that of “hot-deformed magnets”. The main phase, Nd2Fe14B, consists of flat plate-shaped, with the c-axis oriented along the thickness direction of the plate. The expansion of the grains into the (001) plane is the most energy-efficient configuration, as it has the lowest surface energy based on the crystal structure of Nd2Fe14B. Despite these microstructural similarities, it is impossible to get excellent magnetic properties in the case of the as-cast alloy due to the isotropic nature of the grains and the large grain width which is two orders of magnitude longer than that of the hot-deformed magnets. After the hot-deformation was applied to the cast alloy, the coercivity exceeded 800 kA/m, although the grain width remained as large as tens of micrometers. In this study, we investigated the magnetization reversal behavior of deformed magnet prepared directly on as-cast alloys, to shed light on the coercivity mechanism regardless of their unique microstructural features. In addition, this fundamental research is expected to provide guidelines for enhancing the coercivity of Nd-Fe-B magnets.
The 65wt.%Nd2Fe14B-35wt.%Nd70Cu30 ingot was prepared to cylindrical shape with the diameter of 6mm by casting under He gas. The die-upset process was carried out on the casted ingot under 2500kgf at 620℃ to produce an anisotropic magnet. The magnetic properties were measured by VSM(DynaCool, Quantum Design Inc.) and the microstructure characterization was carried out using SEM(JSM-7800F, JEOL Ltd.). The magnetization reversal was investigated using a Magneto-optic Kerr effect microscope(Evico magnetics GmbH) and a magnetic force microscope(DriveAFM, NanoSurf).
Fig.1 shows the cross-sectional backscattered electron (BSE) SEM images of the die-upset sample from the center and edge areas. The pressing direction is indicated by the yellow arrows. The thickness of the platelet-like structure along the c-axis does not have a big difference depending on the observation position of the sample, while the width perpendicular to the c-axis increases at the edge of the sample. The average grain width are 14.1μm and 18.7μm, whereas the standard deviation is 9.0μm and 23.0μm from the center and edge areas of the sample, respectively. The coercivities of the center and edge regions are 998 kA/m and 418 kA/m, respectively. Since the thickness of the plate-like structure is uniform around 1μm, there is a high possibility that the grain width is an important parameter in determining this difference in coercivity.
MOKE microscopy revealed that many periodic magnetization reversal sites were generated in the grains with large widths, and magnetization reversal proceeded rapidly. These periodic magnetization reversal sites were also confirmed in MFM images. In addition, the presence of thick (Nd,Cu)-rich intergranular phase hinders the cascade-like propagation of the magnetic domain wall. Meanwhile, by observation of magnetization reversal behavior using MOKE images at the center area, it was found that the reversed magnetic domain area expands across several grains along the pressing direction, indicating that the grains along the c-axis are magnetically coupled.
The 65wt.%Nd2Fe14B-35wt.%Nd70Cu30 ingot was prepared to cylindrical shape with the diameter of 6mm by casting under He gas. The die-upset process was carried out on the casted ingot under 2500kgf at 620℃ to produce an anisotropic magnet. The magnetic properties were measured by VSM(DynaCool, Quantum Design Inc.) and the microstructure characterization was carried out using SEM(JSM-7800F, JEOL Ltd.). The magnetization reversal was investigated using a Magneto-optic Kerr effect microscope(Evico magnetics GmbH) and a magnetic force microscope(DriveAFM, NanoSurf).
Fig.1 shows the cross-sectional backscattered electron (BSE) SEM images of the die-upset sample from the center and edge areas. The pressing direction is indicated by the yellow arrows. The thickness of the platelet-like structure along the c-axis does not have a big difference depending on the observation position of the sample, while the width perpendicular to the c-axis increases at the edge of the sample. The average grain width are 14.1μm and 18.7μm, whereas the standard deviation is 9.0μm and 23.0μm from the center and edge areas of the sample, respectively. The coercivities of the center and edge regions are 998 kA/m and 418 kA/m, respectively. Since the thickness of the plate-like structure is uniform around 1μm, there is a high possibility that the grain width is an important parameter in determining this difference in coercivity.
MOKE microscopy revealed that many periodic magnetization reversal sites were generated in the grains with large widths, and magnetization reversal proceeded rapidly. These periodic magnetization reversal sites were also confirmed in MFM images. In addition, the presence of thick (Nd,Cu)-rich intergranular phase hinders the cascade-like propagation of the magnetic domain wall. Meanwhile, by observation of magnetization reversal behavior using MOKE images at the center area, it was found that the reversed magnetic domain area expands across several grains along the pressing direction, indicating that the grains along the c-axis are magnetically coupled.
