How can a permanent magnet exist with out superconductivity

windowgobo

i cant quite get my head around how a permanent magnet stays charged for so long. im probably missing something basic.

eZe^

windowgobo wrote: »

i cant quite get my head around how a permanent magnet stays charged for so long. im probably missing something basic.

Ferromagnetism is what causes permanent magnetisation. The spins of unpaired electrons in the presence of external magnetic field will line up, and when the external field is removed hysteresis ensures that a certain portion of the magnetisation will remain.

Professor_Fink

As eZe^ says, ferromagnetism is generally the cause of magnetism in permanent magnets. In these, it is the electron angular momentum which gives rise to the magnetic field. Quantum mechanics insures that there is a minimum possible spin for an electron: it can never stop. The forces acting on the electrons can have a number of different results, depending on the exact configuration of the system, but ferromagnets are the group of systems where these forces result in the electrons having least energy when they rotating in the same direction, and hence their magnetic fields align. Thus in the ground state all of the spins are rotating the same way, so when the system is cooled to 0 temperature, you get a very strong magnet. As the heat rises you still have a large number of spins in their ground state, and so still have a permanent magnet. If you raise the temperature sufficiently, you exceed what is known as the Curie temperature, and the material looses its net magnetic field (assuming there is no external field).

Now, I have mentioned that the forces make the axis of rotation for the electrons align, but you may notice that I didn't specify which direction this is in. This is because the ground state is degenerate, and so there is more than one acceptable solution. If there is no external field, then different regions (known as domains) can lign up differently, leading to a much weaker magnetic field, or none at all. However, when an external magnetic field is applied, the domains which are aligned with it expand their borders, while others shrink. Hysteresis prevents this process from completely reversing when the magnetic field is removed, and so the material gains some net magnetic field. Note that this is not because it wasn't previously magnetic, but rather bacause the different domains were aligned randomly, and hence the net field was very near zero.

windowgobo

thank you for your replies, that helped.