Magnetic Substance

 A substance which is affected by a magnetic field is called a magnetic substance. 

There are three types of magnetic substance. 

1) Diamagnetic substances

     Diamagnetic substances are those substances which are repelled by the magnets, e.g., antimony, bismuth, lead, tin, zinc, mercury, gold, phosphorus, etc. 

2) Paramagnetic substances

     Paramagnetic substances are those substances which are attracted by the magnets, e.g., aluminium, platinum, oxygen, manganese, chromium, etc. 

3) Ferromagnetic substances

     Ferromagnetic substances are those substances which are strongly attracted by the magnetics, e.g., iron, cobalt, nickel, etc. 

 

Diamagnetic substances

Substances belonging to this category are repelled by the magnets and have a tendency to move from the region of stronger magnetic field to the weaker one. 

     Electron revolving round the nucleus is equivalent to a current loop. A current loop gives rise to a magnetic moment. In general, magnetic moments due to different electrons point in all sorts of directions, thus, neutralising each other. 

     Let an external magnetic field of strength B (rightward arrow) be applied in a direction perpendicular to the plane of rotation of the electron. As the electron (charge = - e) moves through this field, it experiences a force F (rightward arrow) = - e (v × B) (both v and B is rightward arrow). The electron which revolves in clockwise direction experiences this force along the radius away from the centre [Fig. 1(i)]. While that rotating in anticlockwise direction has this force acting along the radius towards the centre. Net centripetal force in case (i) decreases. This results in a corresponding decrease in linear velocity of electron and hence a consequent decrease of its magnetic moment from M (rightward arrow) to M - ∆M (both are rightward arrow). Opposite in the case for electron revolving in anticlockwise direction [Fig. 1(ii)]. It has a force F = - e (v × B) acting along the radius towards the centre. This increases net centripetal force and a corresponding increase in its magnetic moment from M to M + ∆M (both are rightward arrow). 

Fig. 1.

      So, for one pair of atoms, which has zero resultant magnetic moment in the absence of external field, now, have a resultant magnetic moment given by

     Change in magnetic moment

            = (M + ∆M) - (M - ∆M) = 2∆M (all are rightward arrow). 

     It is clear that this change in magnetic moment is in a direction opposite to the direction of applied field. It is this resultant magnetic moment which accounts for the diamagnetic behaviour of substance. They have a tendency to move from region of stronger field to that of weaker field. 

Properties of diamagnetic substances

1. Diamagnetic substances are repelled by the magnets. They tends to move from a region of stronger magnetic field to the weaker one. 

2. A bar of diamagnetic substance when suspended in a uniform field comes to rest, at right angles to the lines of force. 

3. A diamagnetic substance, in powdered form, placed in a watch glass over two magnetic poles, separated about 10 cm apart, will get collected in the form of a heap in the centre [Fig. 2]. This is due to the reason that magnetic field in the central region is weaker than that in the corner region. 

Fig. 2. Diamagnetic substance is collected in the weaker field region. 

4. Put a diamagnetic substance, in liquid form in a U-tube whose one limb lies in between the two pole pieces. It will be observed that liquid in limb in between pole pieces, gets depressed while that in the other gets raised [Fig. 3].

Fig. 3. Diamagnetic liquid moves from stronger to weaker field region. 

5. Substance is magnetised in a direction opposite to the direction of magnetising field. 

6. Intensity of magnetisation 'I'm is proportional to the strength of magnetising field. 

7. Permeability of diamagnetic substance is less than one (μ < 1).

8. Magnetic induction B in the diamagnetic substances is less than the strength of magnetising field H (B < H). 

9. Susceptibility 'k' of diamagnetic substances is negative. 

10. Susceptibility of diamagnetic substances is independent of temperature. 

Paramagnetic substances

Paramagnetic materials are those which have at least one unpaired electron. The inner field shells of any elements give rise to diamagnetic character because all the electrons are paired in filled shells. However one or more unpaired electrons in the outmost shell have their magnetic moments (M = - e/2  L) remain uncancelled. 

     In the bulk of material these magnetic moments are all arbitrarily distributed in different directions. This can be explained on the fact that :

     "The thermal energies of atomic magnetic dipoles is more than the interaction energies of various dipoles among themselves."

     Thus, in the absence of external applied magnetic field there is no net magnetic moment in the bulk of paramagnetic sample [Fig. 4].

Fig. 4. Paramagnetic sample in the absence of external field. 

     When magnetic field is applied, the individual atomic dipoles try to allign themselves in the direction of field and net magnetic moment is developed in the sample [Fig. 5].

Fig. 5. Paramagnetic sample in external magnetic field. 

     Before we reach this state, when all the magnetic moments have been arranged due to applied field the intensity of magnetisation is found to be :

     (i) proportional to magnetic intensity in the sample i.e., I ∝ H. 

    (ii) inversely proportional to absolute temperature of the sample i.e., I ∝ 1/T

     Combining factors (i) and (ii), I ∝ H/T

or              I = CH/T         or         I/H = C/T

      Here 'C' is the constant of proportionality

     But     I/H = k (magnetic susceptibility) 

                   k = C/T ; this is called Curie's law of magnetism. 

Curie temperature. As the temperature of a ferromagnetic sample is increased, due to thermal energies, the magnetic moments no longer remain aligned inside the ferromagnetic domains. So, the sample loses its magnetism and behaves like a paramagnetic sample. Thus, 

     "The temperature where a ferromagnetic sample becomes paramagnetic is called curie temperature."

     Curie temperature of a new material is given below :

Material	Fe	Ni	Co	Gd	MnAs	MnB
Curie temperature (K)	1043	627	1388	293	318	5778

Properties of paramagnetic substances

1. They are attracted by the magnets. They tends to move from region of weaker magnetic field to the stronger one. 

2. A bar of paramagnetic substance when suspended in a uniform field comes to rest, along the direction of lines of forces of the field. 

3. Place a paramagnetic substance, in powdered form in a watch glass placed on the two poles of a magnet separated about 10 cm apart. The substance gets collected in the form of two steps at the corners as shown in [Fig. 6]. This is due to the reason that magnetic field in the corner reason is stronger than that in the central region. 

Fig. 6. Paramagnetic substance is collected in stronger field region. 

4. Consider some paramagnetic substance in liquid form put a U-tube whose one limb is kept in between the pole pieces of a magnet. The liquid registers a difference of level in the two limbs [Fig. 7]. Level of liquid in the magnetic field is higher than that outside. This indicates that the substance moves from weaker regions of magnetic field to the stronger one. 

Fig. 7. Paramagnetic substance is collected weaker to stronger field. 

5. They are magnetised in the direction of magnetising field. 

6. Intensity of magnetisation 'I' is proportional to 'H'. 

7. Permeability of a paramagnetic substance is slightly greater than one. 

8. Magnetic induction 'B' is greater than 'H'. 

9. Susceptibility 'k' of paramagnetic substance is positive. 

10. Susceptibility 'k' varies inversely as the temperature of substance. 

                  k ∝ I/T

or              k . T = constant. 

     Here 'T' is the temperature of the substance in Kelvin. 

     This relation is known as "Curie Law".

Ferromagnetic substances 

These substances are strongly attracted by the magnets. 

     A ferromagnetic substance, like iron, is composed of tiny crystal like regions, odd in shape and microscopic in size, that fit together as in a mosaic [Fig. 8]. Within each crystal, there are one or more domains and within each domain there is essentially a perfect alignment of the elementary magnetic moments represented by arrow. This arrangement is of random orientation in an unmagnetised sample. 

Fig. 8. Domains in ferromagnetic. 

     When a magnetic field is applied, the atomic dipoles within a domain may suddenly swing around to line up with the external field, or an already aligned domain may grow in size at the expense of an adjacent domain adversely oriented. In other words, a common boundary between two adjacent domains in the same crystal will move. As the field grows stronger, more and more domains flip around suddenly to line up, while others grow in size, until all the domains are aligned. If a ferromagnetic specimen is heated above a certain temperature, called as 'Curie point', the exchange coupling disappears and the substance becomes paramagnetic. 

Properties of ferromagnetic substances

     The properties of ferromagnetic substances are nearly similar to those of paramagnetic substances, but are exhibited on a magnified scale. 

1. They are strongly attracted by the magnets. They tends to move from region of weaker magnetic field to the stronger one. 

2. A bar of ferromagnetic substance, suspended in a magnetic field, comes to rest along the direction of magnetic field. 

3. A ferromagnetic substance, in powdered form gets collected near the magnetic poles as shown in [Fig. 6].

4. A ferromagnetic liquid moves up in the limb of U-tube placed in a magnetic field as shown in [Fig. 7].

5. They are magnetised in the direction of the magnetising field. 

6. Intensity of magnetisation 'I' is not proportional to 'H'. 

7. Permeability of ferromagnetic substance is much greater than one (μ >> 1).

8. Magnetic induction 'B' is much greater than 'H' (B >> H). 

9. Susceptibility of ferromagnetic substances is positive. 

10. Susceptibility of ferromagnetic substances changes with temperature. 'k' decreases with a rise in temperature. Above a certain temperature called Curie point, the substance starts behaving like a paramagnetic substance. On cooling it below Curie point, the ferromagnetic properties are regained. 

Important Notes

1. A few examples of ferromagnetic, paramagnetic and diamagnetic materials are :

       Ferromagnetic ⟶ Fe, Co, Ni

       Paramagnetic ⟶ Al, Sb, Pt, Cr, Na, etc. 

       Diamagnetic ⟶ Cu, Zn, Bi, Pb, Au, Ag,                                          etc. 

2. All materials have diamagnetic character due to complete inner shells. However the ferromagnetic and paramagnetic effects are much stronger than diamagnetic effects. 

3. The susceptibility of diamagnetic, paramagnetic and ferromagnetic materials vary with absolute temperature as shown below in Fig. 9.

Fig. 9.

Conceptual questions with answers

1. What are the directions of magnetisation of two magnetic substances A (diamagnetic) and B (paramagnetic) when they are placed in same magnetic field ? 

Ans. Substance A which is diamagnetic in nature will get magnetised opposite to direction of magnetising field while B which is paramagnetic in nature will get magnetised in the direction of the magnetising field. Thus, the direction of magnetic moment vectors in the two samples will be opposite to each other. 

2. What type of magnetic substance would you prefer for making permanent magnet ? 

Ans. A ferromagnetic material is most suitable for this purpose. 

• Effect of external magnetic field is strongest. Therefore, the intensity of magnetisation is large. 
• Since it gets magnetised in the direction of magnetic field, therefore, it will remain in equilibrium with the field. 

3. How would you distinguish between a paramagnetic substance and diamagnetic substance by studying their permeability ? 

Ans. Paramagnetic substance have permeability greater than one while diamagnetic have its value less than one. 

4. Why does a rod of diamagnetic substance when suspended in a magnetic field has a tendency to come to rest at right angle to the field ? 

Ans. Diamagnetic substance have a tendency to move from region of strong magnetic field to region of weak field. In other words, they have minimum effect of magnetic field on themselves. When a right angles to the field there is no component of magnetic intensity along their length. Hence they are under the minimum effect of field. 

5. What type of hysteresis loop do you expect in case of a permanent magnet ? 

Ans. In case of a permanent magnet, the value of remanance and co-ericivity are large. Therefore, the hysteresis loop should be tall and wide.