Weiss domains

Problem

Explain in a few lines why a piece of iron is not magnetised in its natural state, and why it becomes magnetised when brought close to a magnet. Also discuss residual magnetisation.

Solution

Iron is divided into Weiss domains, small regions in which the atomic magnetic moments are already all aligned with each other.

In the natural state the domains are randomly oriented: their fields cancel each other out and the piece appears overall unmagnetised.

When a magnet is brought close, the external field makes the domains already aligned with it grow and makes the others rotate until they align: the iron becomes magnetised and is attracted.

Once the external field is removed, part of the alignment remains (residual magnetisation): this is the phenomenon underlying permanent magnets. Heating or striking the material disorders the domains again and the magnetisation is lost.

$\ev{\text{disordered domains} \to \text{aligned by the field} \to \text{residual}}$

Links

Topics: Magnetismo Concepts: Campo magnetico