17 - Magnetic properties of materials

Summary

Individual atoms have magnetic moments due to the orbits and spins of the electrically charged particles within them. Interaction with imposed external magnetic fields tends to produce some ordering of these magnetic moments. But this ordering is opposed by thermal motion, which tends to randomize their orientations. It is the balance of these two opposing influences that determines the magnetization of most materials.

Diamagnetism, paramagnetism, and ferromagnetism

Consider what happens when we place a material in an external magnetic field. According to Lenz's law, any change in magnetic field through a current loop produces an electromotive force that opposes the intruding field. On an atomic level, each electron orbit is a tiny current loop. The external field places an extra force on the orbiting electrons, which causes small modifications of their orbits and a slight magnetization of the material in the direction opposite to the external field (homework). This response is called “diamagnetism” and is displayed by all materials.

In addition, there is a tendency for the tiny atomic magnets to change their orientations to line up with an imposed external field (Figure 17.1). This response is called “paramagnetism.” It gives the material a net magnetic moment in a direction parallel to the imposed external magnetic field. Not all materials are paramagnetic, because in some materials the atoms have no net magnetization to begin with and in others the atomic magnets cannot change their orientations. But most materials are paramagnetic, and their paramagnetism dominates over diamagnetism.