In the previous chapter we studied two contributions to the magnetic moment of atoms – the electron spin and orbital angular momenta. Next we are going to investigate the third (and final) contribution to the magnetic moment of a free atom. This is the change in orbital motion of the electrons when an external magnetic field is applied.

The change in orbital motion due to an applied field is known as the diamagnetic effect, and it occurs in all atoms, even those in which all the electron shells are filled. In fact diamagnetism is such a weak phenomenon that only those atoms which have no net magnetic moment as a result of their shells being filled are classified as diamagnetic. In other materials the diamagnetism is overshadowed by much stronger interactions such as ferromagnetism or paramagnetism.

Observing the diamagnetic effect

The diamagnetic effect can be observed by suspending a container of diamagnetic material, such as bismuth, in a magnetic field gradient, as shown in Fig. 4.1. Since diamagnetic materials exclude magnetic flux, their energy is increased by the presence of a field, and so the cylinder swings away from the high-field region, towards the region of lower field (the north pole in the figure).