North and South

There was a time in primary school when we were first introduced to the curious object that is the bar magnet. Instead of paying attention to actual mechanics behind magnetism, more often than not were we competing to see who could force the north poles of two magnets to touch each other. But what these lessons had taught us are that magnets are indeed curious objects.

Domain theory is one explanation of why some materials are magnetic and some are not. It is not perfect, however, due to modern developments such as the existence of monopoles ( magnets that shockingly only have one pole), but rather like the Bohr model of the atom, it is sufficient up to a surprisingly high standard of physics.

An interesting result of magnetism is that were you to cut a bar magnet in half, two new individual bar magnets would form, each with its own north and south pole. This is an indication that magnetism is based on a dipolar property of the material itself.

A metal is a regular lattice of ions, each which consist of electrons orbiting a nucleus. A spinning electron, in a similar way to an electric current in circuit, is able to induce its own microscopic magnetic field. Many of these magnetic fields add together to form a domain, which is in essence a tiny bar magnet with a north and south pole. Throughout the metal forms a series of domains; the key here is that these domains point in random directions, so that overall the metal is unmagnetised, and the directions of these domains cannot (for the majority of materials) be aligned to any particular direction.

Yet there are some special metals that can align their domains, and we call these ferromagnetic metals. The main ferromagnetic metals are iron (of course), cobalt and nickel, and additionally some rare earth compounds, including gadolinium and dysprosium. An unmagnetised ferromagnetic metal will initially have random domains, but the domains can be aligned by applying an external magnetic field. This explains why rubbing a magnet on an iron object can magnetise it.

In some cases this magnetisation is temporary – the domains return to their random alignments afterwards. In other cases the directions of the domains become locked and the magnetisation is permanent. Often this depends on how the metal is forged. Iron that experiences permanent magnetisation is called hard iron, and for temporary magnetisation, soft iron. The terms ‘hard’ and ‘soft’ in no way refer to the physical strength of the iron, but only to their magnetic properties.

Soft iron has a very significant application in electromagnets, magnets that can be turned on and off. If hard iron were to be used, the magnet would not be able to be turned off since it would remain magnetised permanently.

Domain theory can also explain why physical abuse to a magnet can demagnetise it. The energy you put into the magnet can be transferred to oscillating the ions. The domains may consequently redirect themselves randomly again. This effect can also be imitated by heating the magnet, since the heat energy is transferred to oscillating the ions. Some metals have a Curie temperature above which they lose their magnetic properties.

Yanhao

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