Domains

Figure 2.12 shows a relatively large (i.e. a size order of $10^{-6}$ metres) ferromagnet which contains domains. Domains can be thought of as the magnetic structures which form at small scales within magnets in particular circumstances (Hubert and Schäfer, 1998, 2000). Within these domains the magnetisation is parallel, though the overall magnetisation of any given domain is not in a particular direction. This gives rise to a mean magnetisation of approximately zero across a sample in zero field. Figure 2.13 illustrates an example of domains formed in a sample with a simple closed flux.

At high applied fields -- what defines a high field is dependent on the type, size and shape of the magnet; it must be enough to fully saturate the magnetisation -- no individual domains will form as the overall magnetisation in the sample is homogeneous at these fields; this can be considered to be a single domain. However, when these fields are reduced, other domains can form in order to minimise the overall magnetisation, which often remains at zero field.

Smaller ferromagnets exhibit the property of magnetisation alignment with an applied magnetic field, though below a certain critical size they will not form domains but may form states (see section 2.8.3).

Figure 2.12: A typical ferromagnet in zero field (left) and in an applied field (right)

Figure 2.13: Flux closure (left), and (right) a larger sample attempting to close its flux through domains.