Transformer core properties

Soft magnetic materials (e.g. iron) have domains that easily move into line when the metal is placed in a magnetic field but as soon as the field is removed the domains take on a random pattern again. It returns to being unmagnetized straight away.

Very little energy is required to magnetise a soft magnetic material.

Hard magnetic materials (e.g. steel) have domains that do not easily move into line when the metal is placed in a magnetic field, a strong field is needed for some time, but then, when the field is removed the domains retain the magnetic pattern. The metal stays magnetic for a long time.

A lot of energy is required to magnetise a hard magnetic material. That energy is changed into heat within the material during the magnetism process.

The AC current in the primary coil of the transformer magnetises, demagnetises and remagnetises the core continuously in opposite directions - 100 magnetising and demagnetising events per second!.

Energy is required both to magnetise and to demagnetise the core and this energy is wasted in the form of heat. The energy wasted may be reduced by choosing a material for the core which is easily magnetised and demagnetised, ie a magnetically soft material such as iron, or a special alloy, rather than steel.