The output from a magnetometer, a single value of magnetic moment for the sample, is a combination of the magnetic moments on the atoms within the sample, the type and level of magnetic ordering and the physical dimensions of the sample itself. The moment is also affected by external parameters such as temperature and applied magnetic field.

The ``Intensity of Magnetisation3.2'', $ M$, is a measure of the magnetisation of a body. It is defined as the magnetic moment per unit volume or

$\displaystyle M = \frac{m}{V}$ (3.2)

with units of Am (emucm$ ^{3}$ in cgs notation).[8]

A sample contains many atoms and their arrangement affects the magnetisation. In Figure 3.1(a) a magnetic moment $ \boldsymbol{m}$ is contained in unit volume. This has a magnetisation of $ m$Am. Figure 3.1(b) shows two such units, with the moments aligned parallel. The vector sum of moments is $ 2m$ in this case, but as the both the moment and volume are doubled $ M$ remains the same. In Figure 3.1(c) the moments are aligned antiparallel. The vector sum of moments is now 0 and hence the magnetisation is 0Am.

Figure 3.1: Effect of moment alignment on magnetisation: (a) Single magnetic moment, $ m$, (b) two identical moments aligned parallel and (c) antiparallel to each other.

Scenarios (b) and (c) are a simple representation of ferro- and antiferromagnetic ordering. Hence we would expect a large magnetisation in a ferromagnetic material such as pure iron and a small magnetisation in an antiferromagnet such as $ \gamma$-Fe$ _{2}$O$ _{3}$.

Dr John Bland, 15/03/2003