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I know that when a material is magnetized, the electrons of the atoms get aligned in the same way, they point to the same direction. But then progressively they demagnetize. So, how can permanent magnets mantain their magnetism? What property do they have to do this?

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It is extremely helpful to always remember that electrons (like protons and neutrons) are permanent magnetic dipoles (NIST https://physics.nist.gov/cgi-bin/cuu/Value?muem). So how does a macroscopic field form?

If one want to build up a macroscopic electric field, one hasto separate charges; induce an electron surplus on one side and an electron deficiency on the other.
If I want to build up a macroscopic magnetic field, I have to align the magnetic dipoles of the subatomic particles in one direction, which is done, for example, by an external magnetic field.

And then there is the phenomenon of self-holding that some materials have. Once aligned, the magnetic dipoles hold each other in position. Imagine small bar magnets that are all more or less aligned in the same direction. Then you have a permanent magnet.

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I'll answer with what I can remember. At firsr a permanent magnet has a hard magnetization which means that usually present a very large hysteresis curve (low B, high H$_c$).

It means that the magnetized state is steady and must be maintained somehow by the material itself, as no external field or current is required to magnetize it. This is, if I recall correctly, done by internationale structure; the spin interactions/orbit generates some magnetic field, the electron itself/its spin and the nucleus spin/state as well. The sum of all these générales a field such that it maintains the magnetization over a very long period of time. Hiwever all this depends strongly on the material physical en electronic structures (such as ferromagnets) and the maximum field/magnetization these can have is limited.

Hope it answers your question

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