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When researching about the Curie-Temperature and magnets in general, something got me confused. What is the difference between a ferromagnet and a permanent magnet? In a ferromagnet the spins are already aligned (not like paramagn.)Isn´t that already a permanent magnet? Because my sources say, that ferromagnets have to be polarized to become permanent magnetic. Can someone clear me up?

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The words ultimately refer to the same "objects" but they describe different aspects of them. "Permanent magnets" are objects and they're defined by the external property (what they look like from outside) that the magnetic field remains nonzero around these objects without any activity.

On the other hand, "ferromagnets" are materials and the focus is on the microscopic explanation of their ability to keep the magnetic field nonzero (and indeed, it's due to the aligned electron spins).

If one is a bit more detailed, the known "permanent magnets" don't have to be made of "ferromagnetic" materials. There is one more option, the "ferrimagnetic" materials. They have permanent magnetic fields, too. Like antiferromagnets, ferrimagnets have spins oriented in alternating directions. But unlike antiferromagnets, "ferrimagnets" don't see the perfect cancellation because the up- and down-spins have a different strength (different atoms or positions of atoms). So they have a residual nonzero permanent magnetic fields just like "ferromagnets".

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  • $\begingroup$ Thanks. Already knew about ferrimagnets, but from the way, i understood it, a ferromagnet has to be magnetized first, so that all the Weiß-Domains align and the remanence causes the permanent magnetic field. $\endgroup$ – Magnetar Feb 4 '16 at 10:57
  • $\begingroup$ Thanks. Already knew about ferrimagnets, but from the way I understood it, the Weiß-Domains must first align as well, due to an external field, for the ferromagnet to become permanent magnetic (remanence). $\endgroup$ – Magnetar Feb 4 '16 at 10:59
  • $\begingroup$ The point is that you can't really "fully demagnetize" a ferromagnetic material everywhere. When a ferromagnetic material is no overall magnetic field, it only means that it's divided to lots of small domains which are individually magnetized but in total, they mostly cancel. Small enough domains of a ferromagnetic material are always "permanent magnets". And yes, I think that it's the same for ferrimagnets. The residual field may also differ in small domains but they may be aligned. $\endgroup$ – Luboš Motl Feb 4 '16 at 11:13

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