ferromagnetism

This is the strong magnetism everyone knows: the fridge magnet, the iron nail that a magnet grabs and that can itself become a magnet. Unlike the feeble diamagnetism and paramagnetism, ferromagnetism is thousands of times stronger and, crucially, it can stay switched on after the applied field is gone. Only a few elements do it at room temperature, chiefly iron, cobalt, and nickel, plus some of their alloys and compounds.

The secret is a quantum effect called the exchange interaction. In these particular metals it is energetically favorable for neighboring atomic moments to point the same way, so vast numbers of unpaired electron spins spontaneously lock parallel with no field needed at all. Turn on even a small external field and the alignment snowballs until nearly every moment points together; push harder and you reach magnetic saturation, where no more moments are left to align. The ratio of the internal field to the applied field is the permeability, and for ferromagnets it is enormous, which is why an iron core dramatically boosts an electromagnet or transformer.

Because the alignment is a cooperative lock rather than a fight against heat, a ferromagnet holds its magnetization, which is what makes permanent magnets and magnetic recording possible. But the cooperation is fragile to temperature: heat it past the Curie temperature and the exchange lock breaks, and the material collapses into an ordinary paramagnet. A useful honesty check: strength here means the moments cooperate, not that the atoms are individually special; iron and its non-magnetic neighbor manganese have similar atomic moments, but only iron has the crystal spacing that makes the exchange interaction align them.