The shape anisotropy of permanent magnet with certain shape (non-spherical) is related to the demagnetic field of permanent magnet.
As is known to all, N and S poles will always appear in a block or cylindrical permanent magnet body, and the presence of magnetic poles will generate magnetic fields around and inside the magnet. The magnetic field generated by a permanent magnet pole always runs from the N pole to the S pole, both around and inside the magnet. The demagnetization field inside the permanent magnet is represented by Hd, and its direction is from the N pole to the S pole. The direction of the magnetic moment of the permanent magnet, Mm, or the direction of magnetization, is from the S pole to the N pole. The Hd inside the permanent magnet is opposite to the direction of the magnetization M and plays the role of demagnetization, so it is called demagnetization field.
The demagnetic field intensity of permanent magnet is related to the shape and size of permanent magnet. For example, for a circular permanent magnet, the demagnetic field intensity varies with the gap size of the circular permanent magnet (i.e. the length of the permanent magnet). When it is magnetized along the axis of the circular permanent magnet ring, no demagnetizing field is generated. When half of the circular permanent magnet is cut off, the demagnetic field of the half ring is Hd. The shorter a circular permanent magnet is cut, the greater the absolute value of its demagnetic field Hd. The increasing sequence of the absolute value of the demagnetization field is Hd (e)> Hd (d) >Hd (c) >Hd (b) >Hd (a)→0.
It can be inferred that when the coercivity of permanent magnet Hcj is not large enough, if the permanent magnet is made thin and magnetized along the thin direction, it may not be filled because the demagnetizing field is too large.