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Thursday, 1 August 2019

Classification of Magnetic Materials

In any material all the molecules contain electrons orbiting round the nucleus. The electron flow is taken as current flow as revolutions of electrons in various orbits is considered as flow of circulating currents around the nucleus. All the electrons around the nucleus do not revolve in the same orbit but revolve in different orbits. These orbits are equivalent to circulating current and so develop an magneto motive force. In most of the materials the direction of motion of the electrons in the various orbits is such that they develop magneto motive force in opposite directions thus cancelling each other.

In the case of magnetic material the magneto motive forces developed act in the same direction. In magnetic materials such as steel, there are a number of un-neutralised orbits such that a resultant M.M.F. axis exists which produce magnetic pole called the magnetic dipoles 'A' which is a tiny magnet having North and South poles on the tips of both ends. In an unmagnetic pole axis lie along continuous closed paths and so is not characterised by their relative permeabilities. In accordance with the value of relative permeability, the materials may be classified in the following ways.



(a) Ferro magnetic materials.
(b) Para magnetic materials.
(c) Anti ferro materials.
(d) Ferric materials.
(e) Dia magnetic materials.

The dipole configuration in the magnetic materials is shown in figure and Lines of forces in magnetic material is shown.

(a) Ferro Magnetic Materials :


Ferro magnetic materials have parallel oriented dipoles. These materials are generally crystalline solids. The permanent atomic dipoles are aligned parallel to each other within groups called 'domains' 1,2,3, etc., as shown in the Figure.
Domains in Material

Thus each domain is completely magnetised. But in unmagnetised state, the various domains do not have any magnetisation. A few examples of ferro magnetic materials are iron, Nickel and Cobalt. A weak external magnetic field cannot bring orientation of the domains. But when it is increased, the domains orient themselves such t hat their resultant magnetic field coincides with the external magnetic field and the material develops strong magnetic field. The ratio of strengthening of the internal magnetic field decreases with the increase in the applied magnetic field beyond magnetic saturation point.

When a magnetic material is magnetised gradually in the beginning the flux density is due to the external field only. This is because the domains of the ferro magnetic material do not orient themselves parallel to the applied field. Thus the permeability is constant upto the point 'X' and this is called initial permeability. High values of initial permeability may be obtained when the materials have high saturation and low coercive force. Between the points 'X' and 'Y', the relative permeability μ gradually increases as the external field increases. The axes of the domains have come parallel to the external field completely. After this any further increase in the external magnetic field does not cause any appreciable increase in flux density. That is, the permeability of the material starts decreasing after the point corresponding to 'Y'.

Table gives the maximum permeability's saturation level and coercive force for different Ferro magnetic materials.

SL No
Materials
Approx. %age composition
Fe      Ni        Cu      Other
Maximum permea-bility
Saturation flux density
Coerective Amp/m
1
Cold Rolled Steel.
98 5
-
-
-
2000
2.1
144
2
Iron
99.91
-
-
-
5000
2.15
80
3
4% Silicon iron
96
-
-
4 Si
7000
2.0
40
4
Grain oriented (S-Fe)
97
-
-
3 Si
3000
1.97
12
5
Mumetal
18
75
-
2 Cr
5 Cr
100000
0.65
4
6
78 Permalloy
21.2
78.5
-
0.3 Mn
100000
1.6
4

Para Magnetic Materials :


In the para magnetic materials the interaction between neighbouring dipoles are negligible as the dipoles present in this type are oriented at random. Therefore, the resultant magnetic field is negligible. These materials are freely attracted by magnets. When magnetised, the dipoles orient in parallel to the applied magnetic field but the magnetisation 'M' is small. The relative permeabilities of such materials are taken as unity. The para-magnetic susceptibility varies inversely with the absolute temperature. The relative permeabilities of the para magnetic materials are slightly greater than unity. Aluminium, platinum and oxygen belong to this category. Table 4.2 shows susceptibility of some para magnetic materials.

Susceptibility of Para Magnetic Material at Room Temperature:


Material
X = r-1
r
Air
0.038 x 10-5
1.00000338
Aluminium
2.3 x 10-5
1.003023
Ebonite
1.4 x 10-5
1.000014
Liquid Oxygen
340 x 10-5
1.0034
Nitrogen
0.0013 x 10-5
1.000000013
Oxygen
0.19 x 10-5
1.0000019
Platinum
36.0 x 10-5
1.0036
Tungsten
7.6 x 10-5
1.000076
CuO
580 x 10-5
1.0058
MnSO4
360 x 10-5
1.0036
Fe2O2
140 x 10-5
1.0014
FeCl2
370 x 10-5
1.0037
NiSAO4
120 x 10-5
1.0012



Antiferro Magnetic Materials:


In this type the neighbouring moments are aligned anti parallel and few examples of such materials are Mn, Cr, Ferros and nickel oxides etc. The curie temperature of such materials is given in Table below. The phenomenon is not important because of its low value of susceptibility.

Crystal
Curie Temp Tc (oK)
MnO
122
FeCl2
23.5
FeO
198
NiO
523
CrSb
1000

Ferri Magnetic Material:


Some alloys like XOFe2O2 with X = Mn, Co, Ni, Cu, Mg, Zn, Cd or Fe++ exhibit the properties of magnetization and are called Ferri magnetic materials. In the specific manganese Ferrite the ratio of mixture of manganese oxide to iron is 1:1. They are used widely in Computers and microwave equipments. It has low eddy current losses at high frequencies. The resistivity of Ferrite is very much higher than Ferro magnetic materials.

Dia Magnetic Materials :


The dipoles are absent in these materials and therefore the materials are weakly repelled by a magnet. The intensity of magnetization in them acts in opposition to the applied magnetic field. When an external magnetic field is applied to a dia magnetic material it induces a magnetization 'M' in opposite direction to the applied field intensity M. This means the susceptibility (K) of a dia magnetic material is negative and the permeability is slightly less than unity. Copper, gold, silver, bismuth, Antimony, Phosphorous, Carbon dioxide, Nitrogen etc., belong to this category. Susceptibility of some of the dia magnetic materials are shown in Table.

Susceptibility of Dia Magnetic Material



Material
K = (r-1)
It
1
Graphite
- 1.2 x 10-5
0.999880
2
Copper & Water
- 0.9 x 10-5
0.999991
3
Germanium
- 0.8 x 10-5
0.999992
4
Silicon
- 1.7 x 10-5
0.999983
5
Glass
- 1.3 x 10-5
0.999987



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