Ferromagnetic substances

The substances which when placed in a magnetic field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances. (1) Each atom of ferromagnetic material posseses permanent magnetic dipole moment before application of external magnetic field. (2) The unpaired electron of one atom interacts with electron of neighbouring atom through quantum mechanical exchange interaction and align in a common direction over a small volume of material. These small volume of uniform magnetisation are called domains which are small in size $\left(10^{-18} \mathrm{m}^{3}\right)$ but contain large number of atomic dipoles $\left(\sim 10^{11}\right)$ (3) In absence of external field the domains may be randomly oriented so that their resultant magnetic dipole moment in any direction is zero. (4) If presence of external field the magnetic dipole moment increases due to : (a) Displacement of boundaries of domains. Here size of domain with magnetic dipole moment parallel to applied field increases while for others it decreases. (b) Rotation of domains. Here domains rotate until their magnetic dipole moments are aligned parallel to direction of external field. (5) In presence of weaker fields displacement of boundaries takes place while in stronger fields rotation of domains takes place.             (6) When external magnetic field is removed substance is not totally demagnetised and some magnetism is left in specimen. (7) On heating it loses properly of ferromagnetism at certain temperature called Curie temperature and becomes paramagnetic. On cooling it becomes ferromagnetic again. The Curie temperature for iron is $770^{\circ} \mathrm{C}$. (8) Ferromagnetic substances are paramagnetic substances which acquire very high magnetism in external magnetic field. The two differ in the order of their intensity of magnetism. Properties of Diamagnetic, paramagnetic & Ferromagnetic Materials