Ferromagnetism is the existence of the spontaneous magnetization ,even in the absence of an external magnetic field
Internal magnetic field in ferromagnetism may be hundred or thousand times greater than that of diamagnetic and paramagnetic material
Relation between I and H magnetization intensity and magnetic field is not linear. I and H are no longer have direct proportionality in case of ferromagnetic materials .hence magnetic susceptibility is very large but no longer constant
Even in the absence of external field some ferromagnetic material exhibits large magnetization and can become permanent magnetized
Some of the elements exhibiting ferromagnetic properties at room temperature are iron ,nickel,cobalt and gadolinium
Because of complicated relation ship between I and H in case of ferromagnetic material, it is not possible to express I as a function of H
So when a piece of unmagnetized iron is brought near a magnet or is subjected to the magnetic field of an electric current, the magnetization induced in iron by the field is described by a magnetization curve obtained by plotting the intensity of magnetization I against the field strength H
Ferromagnetism can occur only in paramagnetic material i.e molecule and atoms of a ferromagnetic material also has unpaired electrons and hence non -zero permanent magnetic moment
all ferromagnetic materials are composed of many small magnets or domains ,each of which consists of many atoms within a domain. Size of a domain is usually microscopic
Within the domain, all magnetic moments are aligned ,but the alignment of magnetic moments varies from domain to domain which result in zero net magnetic moment of the macroscopic piece of material as a whole shown below in fig 2(a)
when the substance is placed in an external field ,magnetism of substance can increase in two different ways (i) By the displacement of the boundaries of the domain where domains oriented favourably with respect to the external field increase in and those oriented opposite to the external field are reduced in size as shown in fig 2(b) (ii) By the rotation of domain that is the domain rotate until their magnetic moments are aligned more or less in the direction of the externally applied magnetic field
In presence of week magnetic field material is magnetized mostly by the displacement of the domains and in presence of strong fields magnetization takes place mostly by the rotation of the domains.
In case of ferromagnetic materials on removal of the external magnetic field ,material is not completely demagnetized and some residual magnetization remains in it
Every ferromagnetic material has a critical temperature known as curie temperature (Tc) above which material becomes paramagnetic and this transition of material from ferromagnetic to paramagnetic is a phase change or phase transition analogous to those between solid, liquid and gaseous phases of the matter
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