Diamagnetism and Paramagnetism

As explained in Section 3.1.2 the intensity of magnetisation depends upon both the magnetic moments in the sample and the way that they are oriented with respect to each other, known as the magnetic ordering.

Diamagnetic materials, which have no atomic magnetic moments, have no magnetisation in zero field. When a field is applied a small, negative moment is induced on the diamagnetic atoms proportional to the applied field strength. See Figure 3.2(b). As the field is reduced the induced moment is reduced.

Figure 3.2: Typical effect on the magnetisation, $M$, of an applied magnetic field, $H$, on (a) a paramagnetic system and (b) a diamagnetic system.

In a paramagnet the atoms have a net magnetic moment but are oriented randomly throughout the sample due to thermal agitation, giving zero magnetisation. As a field is applied the moments tend towards alignment along the field, giving a net magnetisation which increases with applied field as the moments become more ordered. See Figure 3.2(a). As the field is reduced the moments become disordered again by their thermal agitation. The figure shows the linear response of $M$ vs $H$ where $\mu H \ll kT$.