exchange interaction

Why should one atom's tiny magnet care which way its neighbor points? The magnetic pull between two atomic needles is far too feeble to matter at everyday temperatures — a real magnet would fall apart from heat in an instant. Something much stronger must be doing the coordinating. That something is the exchange interaction, and it is the true engine behind every form of magnetic order.

It is not a magnetic force at all. It comes from two quantum rules colliding: identical electrons are forbidden from being in exactly the same state, and like charges repel. When two electrons keep their spins parallel, the no-same-state rule forces them to stay farther apart, which lowers their electrical repulsion and so lowers their energy. The upshot is that whether spins prefer to align or anti-align is decided by ordinary electric energy, dressed up by a deep quantum bookkeeping rule — hugely stronger than direct magnetism.

The exchange interaction matters because it explains the central puzzle of magnetism: how moments organize themselves at all. Its sign — favoring parallel or antiparallel — picks ferromagnetism versus antiferromagnetism. The common misconception is that magnets hold together by magnetic attraction between atoms. They do not; the binding glue is electric repulsion routed through quantum statistics, with genuine magnetic forces playing only a minor supporting role.