Some of the most important Physics Topics include energy, motion, and force.
What do you Mean by Paramagnetic Materials? Describe its Different Properties.
Depending on the behaviour of a material placed in an external magnetic field, Michael Faraday first classified all materials into three groups:
- paramagnetic material,
- diamagnetic material and 0 ferromagnetic material.
Paramagnetic Material
The materials attracted feebly by a strong magnet are known as paramagnetic materials. Paramagnetic materials may be solid, liquid or gaseous.
Example: Solid elements like aluminium, potassium, platinum, sodium, tin, manganese, etc., copper sulphate, ferric chloride salts and their aqueous solutions, gaseous materials like oxygen, air, etc.
Relative magnetic permeability of a paramagnetic material varies from 1 to 1.001 and its magnetic susceptibility has a very small positive value (10-4 or less).
Properties:
i) When a paramagnetic material is placed in a non-uniform magnetic field, it moves gradually from the weaker part to the stronger part of that magnetic field, i.e., the material is attracted feebly by the magnet.
ii) If a paramagnetic material is placed in a uniform magnetic field, the lines of force close up a little and pass through the material. As a result, the lines inside that material get crowded slightly. For a sphere, they are shown in Fig.
So, the magnetic field (B) increases slightly inside a para-magnetic material.
iii) A small number of molecules of a paramagnetic material are magnetic dipoles. When heated, random thermal motion of these dipoles increases, and hence their magnetic alignment is disturbed. As a result, both magnetic susceptibility and magnetic permeability decrease. Magnetic susceptibility per unit mass of a paramagnetic material, \(\chi\) = \(\frac{k}{\rho}\) (ρ = density) is inversely proportional to the absolute temperature; i.e., \(\chi\) = \(\frac{C}{T}\) (for a particular material C is constant). This is known as Curie law. Experimental observations show that Curie law is applicable only for paramagnetic gases. In case of paramagnetic solids, Curie- Weiss law is applicable. According to this law, magnetic susceptibility, \(\chi\) = \(\frac{C}{T-\theta}\)
where, θ is a specific temperature known as Curie tem-perature of that paramagnetic material. The values of Curie temperature are very small for almost all paramagnetic materials.
iv) If a liquid is poured through one of the limbs of a vertical U-tube then the liquid rises up to the same level in both limbs. If the liquid is paramagnetic in nature and if one of the limbs of the U-tube is placed between the two poles of a strong electromagnet, it is observed that the liquid rises in that limb [Fig. 2.22]. This is due to attraction of paramagnetic liquid by a magnet.
Diamagnetic Material
The material repelled feebly by strong magnets are known as diamagnetic materials. Diamagnetic materials may be solid, liquid or gaseous.
Example: Antimony, bismuth, zinc, copper, silver, gold, lead, mercury, water, hydrogen, etc.
Relative permeability of a diamagnetic material is slightly less than 1 and its magnetic susceptibility is slightly negative (≥ 10-4).
Properties:
i) If a diamagnetic material is placed in a non-uniform mag-netic field, it tries to move from the stronger region to the weaker region of the magnetic field. So, the material is fee-bly repelled by a magnet.
ii) If a diamagnetic material is kept in a uniform magnetic field, the lines of force move away a little. As a result, the lines are more sparse inside the material than outside. For a sphere, they are shown in Fig.
So, the magnetic field (B) decreases slighly inside a dia-magnetic material.
iii) In a diamagnetic material, almost none of the molecules are magnetic dipoles. So the random molecular motion due to increase of temperature has negligible effect on the magnetic properties. Increase of applied magnetic field also has no effect on the magnetic alignment of molecules. As a result, magnetic susceptibility of a diamagnetic material does not depend on the applied magnetic field and temperature.
iv) Theoretically it is proved that, diamagnetism is present in every material. Diamagnetic property of a material is very much weaker than its paramagnetic and ferromagnetic properties. As a result, in spite of the magnetic properties present in paramagnetic and ferromagnetic materials, dia-magnetism remains dormant in them. Hence, it can be said that diamagnetism is the most fundamental magnetic property.
v) If some diamagnetic liquid be poured in a U-tube and if one of the limbs is now placed between the pole-pieces of a strong magnet, the level of the liquid in the limb falls [Fig.]. This is due to the repulsion of diamagnetic liquid by the magnet.
Ferromagnetic Material
Five metals—iron (Fe), nickel (Ni), cobalt (Co), gadolinium (Gd), dysprosium (Dy) and alloys like steel, etc. are attracted strongly by any magnet. Even if the inducing magnetic field is removed, these materials can retain some induced magnetism. These are known as ferromagnetic materials.
Relative magnetic permeability of a ferromagnetic material is usually very high (102 to 106) and its magnetic susceptibility is also very high and positive (200 to 2000 approximately).
Though the magnetic properties of ferromagnetic materials are just like that of paramagnetic materials they show stronger paramagnetism and hence they are placed in a separate group. Heu-sler alloy is a special kind of ferromagnetic material, because none of its components (Cu, 64% , Mn 24% , Al 12% ) are ferromagnetic in nature.
Properties:
i) Generally, ferromagnetic materials are solid and crystalline.
ii) If a ferromagnetic material is placed in a non-uniform magnetic field, it moves very fast from the weaker to the stronger region of the field and sets itself parallel to the direction of the magnetic field. So, ferromagnetic materials are very strongly attracted by a magnet.
iii) If a ferromagnetic material is placed in a uniform magnetic field, the lines of force crowd too much within the material than in air.
So, the magnetic field (B) is very high inside a ferromag-netic material. The value of jn of a ferromagnetic material is also very high. For example, relative magnetic permeability of iron is approximately 2000, for nickel it is 300 and for cobalt it is 250. Magnetic susceptibility (k) of ferromagnetic material has high positive value.
iv) Magnetic permeability and magnetic susceptibility of a ferromagnetic material are not constants. They change with the change in the magnitude of applied magnetic field.
v) Magnetic susceptibility of ferromagnetic materials changes with the change in temperature. If the magnetic field is weak, magnetic susceptibility increases with increase in temperature. In a stronger field, the magnetic susceptibility decreases with increase in temperature.
vi) If a ferromagnetic material is heated gradually, at a particular temperature the material loses its ferromagnetic property completely and is converted into a paramagnetic material. The transition temperature at which a ferro-magnetic material is converted into a paramagnetic one is called Curie point or Curie temperature for that mate-rial. Above Curie point, magnetic susceptibility obeys Curie-Weiss law. Curie point of some materials are men-tioned in Table-2.
Table-2
vii) Even if the inducing magnetic field is removed, a ferromag-netic material can retain some induced magnetism, i.e., a ferromagnetic material possesses magnetic retentivity. It should be mentioned here that, though a ferromagnetic material can be converted into a paramagnetic one by heating above Curie temperature, ferromagnetism and paramagnetism are two totally seperate phenomena. A paramagnetic material cannot be converted into a ferro-magnetic material by cooling. The change is irreversible.
Magnetic screen: Magnetic screen is an arrangement which works on the property of ferromagnetism and with this, a region can be kept absolutely free from the influence of any external magnetic field.
If a magnetic needle is kept in front of any pole of a bar magnet, the needle gets deflected. But if a plate of soft iron be placed in between them, the magnetic needle suffers no deflection. This is due to the fact that magnetic permeability of the plate of soft iron is much greater than that of air. Hence, the magnetic lines of force emerging from the bar magnet try to traverse maximum distance through the plate of soft iron, and hence the lines of force cannot cross over to the other side of the plate [Fig.]. Therefore, the region beyond the plate remains free from the influence of the magnetic field.
Similarly, if a soft-iron ring be placed in front of any pole of a bar magnet, most of the lines of force pass through the iron-ring just avoiding the air gap inside the ring [Fig.]. Since no line of force enters this region A inside the ring, it remains free from the influence of all external magnetic fields. If a magnetic needle is placed in that region, it sets itself at rest in any direction. So, a magnetic material used to make a particular region free from the influence of any external magnetic field, is called magnetic screen.
Magnetic screens made of soft iron are used to protect delicate measuring instruments like galvanometers, ammeters, etc. from external magnetic fields. To make a costly wrist watch antimag- netic, a soft iron ring is fitted around it.
Comparison of Ferromagnetic, Paramagnetic and Diamagnetic Materials
Ferromagnetic Material
- Attracted strongly by magnet.
- Generally crystalline solid.
- When placed in a magnetic field, lines of force crowd too much inside the material.
- In a non-uniform magnetic field, this material moves to the stronger region from the weaker region of the field quickly and sets itself parallel to the magnetic field.
- Relative magnetic permeability (μr) is of high order (usually 102 to 106).
- Magnetic susceptibility (k) is a large positive number (200 to 2000 approximately).
- Change in magnetic susceptibility with the change in temperature is complicated in nature.
- Shows retentivity.
- Has a characteristic Curie point.
- Both magnetic permeability (μ) and magnetic susceptibility (k) change with the change of the external magnetising field.
- Magnitude of the magnetic field B Inside is much greater than that out-side.
- Possesses hysteresis property.
Paramagnetic Material
- Attracted feebly by magnet.
- May be solid, liquid or gas.
- When placed ¡n a magnetic field, lines of force crowd inside the material to a slight extent.
- In a non-uniform magnetic field, this material moves rn the stronger region from the weaker region of the field slowly and sets itself parallel to the magnetic field.
- μr is slightly greater than 1 (usually 1 to 1.001).
- Magnetic susceptibility (k) is a small positive number ( 10-4 or less).
- With the increase in temperature, magnetic susceptibility (k) decreases.,
- Has no retentivity.
- Has no Curie point.
- Values of μ and k do not depend on the external magnetising field.
- Magnitude of the magnetic field B is slightly greater inside the material.
- Does not possess hysteresis.
Diamagnetic Material
- Repelled feebly by magnet.
- May be solid, liquid or gas.
- When placed in a magnetic field, lines of force are more sparse inside than outside the material.
- In a non-uniform magnetic field, this material tends to go to the weaker region from the stronger region of the field and sets its largest axis normal to the direction of the magnetic field.
- μr is slightly less than 1.
- Magnetic susceptibility (k) is a small negative number (-10-4 or less).
- Magnitude of magnetic susceptibility does not depend on temperature.
- Has no retentivity.
- Has no Curie point.
- Values of μ and k do not depend on the external magnetising field.
- Magnitude of the magnetic field B is less inside the material.
- Does not possess hysteresis.