The direction of magnetization in which the magnetic field becomes the strongest in the x-axis direction in the spherical coordinate system is used. Its magnetic field depends on the φ direction in the spherical coordinate system, i.e., strongest at the top (φ = 0) and the bottom (φ = π), and the side (φ = π ⁄ 2) becomes the weakest. Therefore, when segmenting, it was divided to include the top and the bottom parts. In addition, in order to make it an optimal segmentation, it is effective to make the range of the top and bottom divisions wide, the sides narrow, and the direction of magnetization close to the top and the bottom directions. By optimizing in this way, the magnetic field is larger than the equal division. For example, it was about 1.09 times stronger in three divisions. This was confirmed by the experiment. However, there is a problem that the demagnetic field in the center becomes strong by the optimizing. The type of division which is strong in this problem is also introduced.
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