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Ferromagnetism

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Before the 19th century, scientists used to believe that magnetic properties was confined to a few materials like iron cobalt and nickel. But in 1846 Curie and Faraday discovered that every material in the universe is magnetic to some extent. Thus, magnetic materials were categorized mainly into two groups:
  • Weak magnetic materials come under Diamagnetic and Paramagnetic materials
  • Strong magnetic material are Ferromagnetic materials.

Let's see about the Ferromagnetism acting in ferromagnetic materials.

Ferromagnetic Definition

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Ferromagnetic materials are materials strongly attracted by magnets. Their relative permeability values are very high ( $\mu_r$ = 10 to 10,000 ). In other words, they possess all the paramagnetic properties but to a very high degree.
Example: Iron, nickel and cobalt.

Ferromagnetic Materials

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In materials like iron, cobalt, nickel and some alloys like alnico (Al, Ni and Co), there is a strong resultant magnetic moment in the atoms. In fact, there will be localized regions known as 'magnetic domains' in which all the atomic magnets are already oriented in a particular direction. But these 'domains' may be randomly oriented making the resultant magnetic moment of the material zero. When they are placed in external magnetic field, these domains get arranged orderly in the field direction.

If the field is removed, the domains suddenly don't get disoriented as in the case of paramagnetic materials and the material still remains magnetized permanently to some extent. This nature is called as ferromagnetism.
The net magnetic moment shows the magnetization even in the absence of external magnetic field.
Ferromagnetism Dipole Moment
When a ferromagnetic material is placed in external magnetic field, the lines of force through the material will be very large in number, as if they are sucked into it. As temperature is increased, the material loses its ferromagnetism and at a particular temperature known as Curie temperature the ferromagnetic substance turns paramagnetic in nature.

Magnetic materials gain importance in electrical engineering. They are employed in making electromagnets, transformers, relay systems etc. They are also used in making permanent magnets.
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Magnetic Dipole

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The magnetic field is a dipole with South pole and the North pole. Since opposite ends of magnets are attracted, the north pole of a magnet is attracted to the south pole of another magnet. When a ferromagnetic material is in a magnetic field, the magnetic dipoles align to the applied field, thus expanding the magnetic domains.