Classification of Magnetic Material

There are three types of magnetic materials. They are:

1. Diamagnetic material
2. Paramagnetic material
3. Ferromagnetic material

Diamagnetic material

Those substances which are feebly magnetized in the direction opposite to the applied field are called diamagnetic material. Examples of diamagnetic materials are bismuth, copper, water, mercury, alcohol, argon, gold,tin, mercury, antimony etc. The magnetic moment of atoms of a diamagnetic material is zero. They acquire induced dipole moments when the material placed in an external magnetic field. These moments are in opposite in the direction to the applied field.

Some properties

1. The diamagnetic materials are repelled by magnets.
2. When a diamagnetic liquid in a watch glass is placed over two closely spaced pole pieces of the magnet, it is depressed at the middle while in the case of pole pieces separated by a distance, it rises at the middle. Similarly, when a diamagnetic liquid is placed in a U-tube and one of the limbs of the tube is placed between the two strong pole pieces of magnet, the liquid depressed at that limb.
3. The diamagnetic materials move from a stronger to a weaker field.
4. A diamagnetic rod, freely suspended in a magnetic field, slowly turns to set at right angle to the applied field.
5. Since magnetized is opposite in direction to an applied field, the diamagnetic materials have the small value for the intensity of magnetization, I.
6. The materials have always negative magnetic susceptibility, \(\chi = (\mu_r -1)\) and accounts from -10^-6 to -10^-5.
7. These materials are independent of temperature.

Paramagnetic Material

Those materials which are weekly magnetized in the same direction of the applied magnetic field are called paramagnetic material. The examples of paramagnetic materials are aluminum, chromium, oxygen, manganese, alkali, alkaline earth metal etc.

The paramagnetic materials have permanent magnetic moments. These moments interacts weekly with each other and randomly orient in the different direction.

Some Properties

1. The paramagnetic materials are feebly attracted by magnets.
2. A paramagnetic rod, freely suspended in a magnetic field, aligns along the field.
3. The paramagnetic materials are temperature dependent and follow curve law.
4. The relative permeability is nearly unity than ranges from 1.00001 to 1.003 for common ferromagnetic materials at room temperature. So, the magnetic lines of force inside the material placed in a magnetic field are more than that outside it.
5. The susceptibility of paramagnetic substances has small positive value.

Ferromagnetic Material

The ferromagnetic materials are highly magnetized in a magnetic field. The examples of ferromagnetic materials are iron, nickel and cobalt, and their alloys such as alnico. Gadolinium and dysprosium are ferromagnetic at low temperature.

Some Properties

1. Ferromagnetic materials are highly attracted by magnets.
2. Ferromagnetic materials more from weaker to stronger field.
3. A ferromagnetic rod, freely suspended in a magnetic field, turns fast to set along the applied field.
4. The magnetic susceptibility is positive and very high and varies with applied field.
5. The relative permeability is very high in the order of 1000 to 100,000.
6. Ferromagnetic dust in a watch glass, placed over two closely spaced pole-pieces of the magnet, increases at the middle, while pole piece is separated by a distance, depresses in the middle.

   Domain Theory of Ferro Magnetism

   

   Each atom of ferromagnetic substance has a permanent magnetic substance; in the unmagnetised state, the atomic and molecular dipoles are arranged in random so the net magnetic moment is zero. There is a strong interaction with neighboring atoms which keeps their magnetic moment aligned parallel in small regions even in the absence of an external field. These small regions with the volume ranging between 10^-12 to 10^-8 m³ are called domains. When the material is placed in an external field B_e, the domains tend to orient themselves parallel to field B₀. As the applied field becomes stronger, the domains, having magnetic moments not aligned with the field, become very small and when the domains fully align to the applied field, the material attains magnetic saturation. On removing the field, the domain walls do not move completely into previous positions. This means material retains a magnetization in the direction of the applied field.

   Hysteresis

   

   

   • Assume a piece of magnetized iron placed in a magnetizing field H.
   • When the value of H is gradually increased from zero value. The magnetic induction B in an iron also increases.
   • When H is increased, B also increases till saturation point P but beyond P if H is increased B remains constant.
   • When the value of H is now decreased, B does not retrace the path PO but at the lower rate and at H being zero, B is not zero but has the finite value represented by OC. The value of B at this point is called retentivity.
   • Further, when H is increased in opposite direction, the value B falls to zero for the magnetizing field to be equal to OD. This value of the magnetizing field is called coercivity of material of specimen.

References

Manu Kumar Khatry, Manoj Kumar Thapa,et.al Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.