Magnetism:
Everyone has seen and handled a magnet in the form of a straight or horseshoe-shaped bar of steel or steel-alloy. The magnet was originally a piece of steel before it was magnetised.
A material called magnetite is a naturally occurring magnet (also called lodestone) which was used at sea for primitive navigation.
A magnet is easily recognised by its ability to attract pieces or iron and steel; and if suspended freely on a piece of string, will swing to align with the earth's own magnetic field.
MAGNETIC THEORIES
If we continue cutting our magnet into smaller and smaller pieces we would eventually arrive at the smallest piece, which would be a molecule and this molecule would be a magnet. Thus the molecular theory of magnetism states that:
• All materials contain molecules with magnetic properties.
• In unmagnetised substances, these molecules are arranged in a random manner and no external magnetic effect is produced.
• When the material is being magnetised, we are aligning the molecules. The number aligned increases, as we further magnetise the specimen and when all are aligned no further increase in magnetisation is possible and the specimen is said to be magnetically saturated.
• In theory all substances could be magnetised, but in practice it is impossible to align the molecules of most substances.
DOMAIN THEORY
In domain theory it is assumed that magnetic materials are composed of tiny individual magnets called domains, a single domain is very small about 1012 atoms.
Considering each atom orbital electrons not only orbit the nucleus but spin axially on their own axis.
In non magnetic materials the same number of electrons spin clockwise as anti clockwise. In magnetic materials more electrons spin one way than the other way
The unbalanced spin creates twists called magnetic moments.
In unmagnetised state the moments of the electrons are in the same direction in a single domain, but the domains produce random pockets of magnetism.
As the magnetic material becomes magnetised the domains become partially aligned. In fully magnetised material all domains become fully aligned.