9.1 Magnetic response

When a material is placed in a magnetic field, it can respond in a number of ways, all of which have their origin in the interaction of the field with the angular momenta of the components of the atoms. This response is measured by the isothermal magnetic susceptibility ? _T, which is defined by

where H is the applied external field and M is the magnetization, defined as the magnetic moment per unit volume. (For anisotropic crystals, the susceptibility is a tensor that is related to the crystal structure. This effect will be ignored.)

There is always a diamagnetic response. In this case, the applied magnetic field induces currents in the electron systems of the atoms with an associated magnetic field that opposes the external field. The induced magnetic moments in a diamagnetic material therefore are in the opposite direction of the applied field, and the diamagnetic susceptibility is negative. The diamagnetic effect is small and essentially independent of temperature and will not be treated here.

The atoms or electrons in a material may have permanent magnetic moments of their own, an is the case for conduction electrons in a metal, or for atoms or ions with an odd number of electrons or partially filled inner shells. A magnetic dipole tends to line up in the direction of an applied field, so the magnetic susceptibility is positive. That is, the moments in a paramagnetic material tend to line up in the same direction as...