Magnetic Dipoles and Magnetic Materials

Every atom in iron acts like a tiny magnetic dipole, a result of its electrons' spins and motion. On their own, these atomic dipoles point in every direction, canceling out. But in ferromagnetic materials, groups of neighboring atoms lock together into regions called magnetic domains, and within each domain the dipoles all point the same way. In an unmagnetized nail, the domains themselves point every which way, so the nail shows no net magnetism even though each domain is already strongly magnetic.

Rub that nail with a magnet and you apply an external field. That field exerts a torque on each domain, nudging the ones that already point roughly the right way to grow at the expense of neighboring domains, and rotating others into alignment. Once most domains point the same direction, the nail itself has a net magnetic field: it has become a magnet. Heat or repeated hammering adds random jostling energy that knocks the aligned domains back out of order, which is why a magnet loses its strength if you heat it past a critical temperature or strike it repeatedly. Not every metal can do this trick. Ferromagnetism depends on a specific atomic arrangement that lets domains lock together and align cooperatively, and only a few elements, iron, nickel, cobalt, and some of their alloys, have it. Most metals, like aluminum or copper, have atomic dipoles that barely interact with neighbors, so an external field cannot organize them into strong domains.

This domain picture sets up the rest of the unit: once you see a magnet as organized atomic current loops rather than some separate magnetic substance, it is a short step to treating current-carrying wires and loops as magnetic dipoles too, and to understanding how an external field can later force those domains to flip and reorganize.