ferromagnetism

Ferromagnetism is the strong, familiar magnetism of iron and a few of its neighbours, the kind that makes a magnet stick hard to a fridge and lets you pick up a chain of paper clips. It is by far the most powerful form of ordinary magnetism, and the only kind strong enough to make a magnet you can hold and feel. It answers, why are some materials, like iron, so strongly and permanently magnetic while most things are not?

Inside a ferromagnet, the tiny atomic magnetic moments do not just point wherever they like; a quantum interaction (called the exchange interaction) makes neighbouring atoms line their moments up in the same direction, forming regions called magnetic domains where trillions of atoms point together. In an unmagnetised piece the domains point every which way and cancel out. Apply an external field and the domains that agree with it grow while others shrink and rotate, until much of the material points one way; the material then becomes a strong magnet. Crucially, in a ferromagnet this alignment can survive after the external field is removed, giving a permanent magnet.

Only a handful of elements are ferromagnetic at room temperature, chiefly iron, cobalt, and nickel, plus certain alloys. Heat destroys the effect: above a temperature called the Curie point, thermal jostling overwhelms the alignment and the material abruptly loses its ferromagnetism, becoming only weakly (paramagnetically) magnetic. This is why ferromagnetism is a property of specific materials in specific conditions, not a universal one.