Each domain is a strong magnet as all atoms or molecules in a domain have same direction of magnetic moment .
Curie Law in Magnetism
The magnetic susceptibility of a paramagnetic substance is inversely proportional to its absolute temperature .
χm ∝ 1 / T ⇒ χm T = constant
where χm = magnetic susceptibility of a para magnetic substance and T = absolute temperature .
Hysteresis
The lagging of intensity of magnetisation ( I ) or magnetic induction ( B ) behind magnetising field ( H ), when a specimen of a magnetic substance is taken through a complete cycle of magnetisation is called hysteresis .
Retentively or Residual Magnetism
The value of the intensity of magnetisation of a material , when the magnetising field is reduced to zero is called retentively or residual magnetism of the material .
Coercively
The value of the reverse magnetising field that should be applied to a given sample in order to reduce its intensity of magnetisation or magnetic induction to zero is called coercively .
Permanent Magnets
Commonly steel is used to make a permanent magnet because steel has high residual magnetism and high coercively .