When you think of magnetic materials, you possibly think of nickel, iron or magnetite. Distinct to paramagnetic materials, the atomic moments in these materials show very strong interactions. These interactions are produced by electronic exchange force. They result in a parallel alignment of atomic moments. The exchange force is a quantum mechanical phenomenon due to the relative orientation of the spins of the two electrons. Ferromagnetic materials show parallel alignment of moments resultant in large net magnetization even in the lack of a magnetic field. The elements Ni, Fe, and Co and most of their alloys are typical ferromagnetic materials: Two distinct characteristics of ferromagnetic materials are there:
(A) Spontaneous magnetization and
(B) The existence of magnetic ordering temperature
The spontaneous magnetization is that exists inside a uniformly magnetized microscopic volume in the absence of field. The magnitude of this magnetization, at 0k, is dependent on the spin magnetic moments of electrons. An interrelated term is the saturation magnetization which we can calculate in the laboratory. The situation magnetization is the maximum induced magnetic moment that can be obtained in a magnetic field. Beyond this field no more increase in magnetization takes place.
Electronic exchange forces in ferromagnetism are very large. Thermal energy ultimately overcomes the exchange and generates a randomizing effect. This takes place at a particular temperature known as Curie temperature. Below the curve temperature, the ferromagnetic is ordered and above it disordered. The saturation magnetization moves to zero at the Curie temperature. A typical plot of magnetization versus temperature for magnetic.
HYSTERESIS: In additional to the Curie temperature and saturation magnetization. Ferro magnets can keep a memory of an applied field also once it is removed. This behaviour is called hysteresis a plot of the variation of magnetization with magnetic field is a hysteresis loop. Another hysteresis property is the coercively of reminisce. This is the reverse field which when applied and then removed, reduces the saturation reminisce to zero. It is always large then the coercive force.