Para and ferro magnetism
Magnetism is a class of physical phenomena that are mediated by magnetic fields. Electric currents and the magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Every material is influenced to some extent by a magnetic field. A material is called paramagnetic, if its molecules / atoms possess permanent magnetic dipole moment. Normally, the molecules are so arranged that their magnetic dipole moments are randomly distributed with zero resultant.The magnetic moment is a property that causes electrons to experience a force when they are placed in a magnetic field.
The magnetic moment (\mu_S) due to spin angular momentum (S) is given by:
ΜS = -e/mS
where e and m are the charge and mass of an electron respectively.
In materials whose atoms have unpaired electrons, the magnetic moments of individual electrons cannot fully cancel out, and so the atoms are left with a resultant magnetic moment. However, the magnetic moments of atoms are aligned in random directions, so the material as a whole does not exhibit magnetism. However, if such a material is placed in an external magnetic field, then the magnetic moments of individual atoms can then align with the external magnetic field, causing the material to become magnetised. The magnetic field produced by paramagnetic materials points along the same direction as the external magnetic field.
Atoms that make up ferromagnetic materials have unpaired electrons in their atoms so each atom has a net magnetic moment. The magnetic moments of nearby atoms tend to become aligned, creating different regions (called domains) in the material, where magnetic moments due to individual atoms are aligned. However, different domains may still have their magnetic moments pointing in different directions.
Behaviour in External Magnetic Fields
Diamagnetic materials align their magnetic fields in the opposite direction to the external magnetic fields.
Paramagnetic and ferromagnetic materials align their magnetic fields in the same direction as the external magnetic fields.
Retention of Magnetism
Diamagnetic and paramagnetic materials lose their magnetization when the external magneic field is removed.
Ferromagnetic materials can retain their magnetization even when the external magnetic field is removed.