Ferromagnetic, Paramagnetic, and Diamagnetic Materials

Every atom has electrons that act like tiny current loops, each producing a small magnetic moment. In most atoms these moments come in pairs that point opposite ways and cancel out. Iron, cobalt, and nickel have unpaired electrons whose moments do not cancel, and critically, the quantum mechanical interaction between neighboring atoms makes it energetically favorable for these moments to point the same direction. That cooperative alignment spreads across billions of atoms into regions called magnetic domains, each domain acting like a tiny bar magnet. In an unmagnetized piece of iron the domains point in random directions and cancel on average, but an external field can swing them into alignment, producing a strong net magnetization that reinforces the field pulling the iron in.

Copper and aluminum are metals with plenty of free electrons, but their electron configurations pair up almost completely, leaving no strong unpaired-spin cooperation to build domains. They are paramagnetic or diamagnetic at best: paramagnetic materials have some unpaired spins that weakly align with an external field and relax back once it is removed, while diamagnetic materials actually develop a faint opposing moment. Neither produces anything close to the domain-driven attraction iron shows. So magnetism is not a generic property of metals, it is a property of specific electronic structures.

Heat enough and even iron loses its magnetism. Above the Curie temperature, thermal vibrations are energetic enough to overpower the cooperative alignment forces holding domains together, so the moments randomize and net magnetization drops to zero. Cooling back below that temperature lets domains reform. This topic sets up the rest of the unit: once you know what makes a material magnetic at all, you can ask how moving charges and currents generate the fields that align these domains in the first place.