The fundamental cause of magnetism, is the movement of electrically charged particles. In some materials this results in the creation of very small magnetised regions called domains. When these domains are randomly orientated, it cancels out any overall magnetic effects within the material. However, the domains can become aligned, if they are acted on by a strong magnetic field. In hard magnetic materials, this alignment is retained, even when the external field is removed and the material becomes permanently magnetised. One process used to magnetise ferromagnetic materials, is to repeatedly draw another magnet along the material in the same direction. Gradually the material becomes magnetised as the magnetic domains become aligned. Another method involves the use of an electrical current, this subject will be considered later.
Magnets can become demagnetised if they are subject to a physical shock such as a hammer blow or to extreme heat. Both of these actions cause the magnetic domains to revert to a random structure.