Page "Ferromagnetism" Paragraph 20

According to classical electromagnetism, two nearby magnetic dipoles will tend to align in opposite directions, so their magnetic fields will oppose one another and cancel out.

However, this effect is very weak, because the magnetic fields generated by individual spins are small and the resulting alignment is easily destroyed by thermal fluctuations.

In a few materials, a much stronger interaction between spins arises because the change in the direction of the spin leads to a change in electrostatic repulsion between neighboring electrons, due to a particular quantum mechanical effect called the exchange interaction.

At short distances, the exchange interaction is much stronger than the dipole-dipole magnetic interaction.

As a result, in a few materials, the ferromagnetic ones, nearby spins tend to align in the same direction.