Classification of magnetic materials based on magnetic moments

CLASSIFICATION OF MAGNETIC MATERIALS BASED ON MAGNETIC MOMENTS

Magnetic materials are classified according to the presence or absence of the permanent magnetic dipoles. Generally, every two electrons in an energy state of an atom will form a pair with opposite spins. Thus the resultant spin magnetic moment is zero. Hence they don't have permanent magnetic moments and they are called as diamagnetic materials. Examples. gold, germanium, silicon, etc.

But in some magnetic materials like iron, cobalt, etc., there exists unpaired electrons. The spin magnetic moment of these unpaired electrons interact with the adjacent atom's unpaired electron spin magnetic moment in a parallel manner resulting in enormous permanent spin magnetic moment. These materials are classified into paramagnetic, ferromagnetic and ferrimagnetic materials with respect to the electron spins.

Classification of Magnetic materials

1. Not having permanent magnet moment: Dia‒magnetic material

2. Having permanent magnetic moment: Para‒magnetic material, Ferromagnetic material, Ferrimagnetic material

1. DIAMAGNETIC MATERIAL

In a diamagnetic material, the electron orbits are more or less random, and mostly all the magnetic moments are cancelled. Similarly all the spin moments are almost paired i.e., they have even number of electrons and has equal number of electrons spinning in two opposite directions as shown in Fig. 1.5. Hence the net magnetic moment in the diamagnetic material is zero. Therefore most of these materials do not have magnetism in the absence of magnetic field.

Effect of external field

When an external magnetic field is applied, the electrons reorient in such a way that they align perpendicular to the field direction and their magnetic moments opposes the external magnetic field. This will reduce the magnetic induction present in the specimen.

Properties

1. They repel the magnetic lines of force (Fig. 1.6)

2. Susceptibility is negative and it is independent of temperature and applied magnetic field strength.

3. Permeability is less than 1.

4. There is no permanent dipole moment, so they are called weak magnets.

5. When temperature is less than critical temperature diamagnetics become normal material.

Examples. Gold, Germanium, Silicon etc.

2. PARAMAGNETIC MATERIALS

In the case of paramagnetic materials, the spins in two opposite directions will not be equal. There exists some unpaired electrons (Fig. 1.7) which gives rise to spin magnetic moment. Hence the resultant magnetic moment will not be equal to zero.

However in the absence of external field the magnetic moments are oriented randomly. Due to its random orientation some magnetic moments get cancelled and the material possess very less magnetisation in it.

Effect of external field

When an external field is applied, the magnetic moments of individual molecules reorient itself along the direction of the magnetic field and the material is magnetised.

Properties

1. The magnetic lines of force pass through the material (Fig. 1.8)

2. Magnetic susceptibility is positive and it is given by

 χ= C / T‒θ

(also called Curie‒Weiss law)

where

C‒Curie constant

T‒ Absolute temperature

θ-Curie temperature

3. Permeability is greater than one.

4. They possess permanent dipole moment.

5. When the temperature is less than curie temperature, paramagnetic materials becomes diamagnetic material.

Examples: CuSO₄, MnSO₄, Platinum etc.

3. FERROMAGNETIC MATERIALS

In a ferromagnetic material the number of unpaired electrons are more. Most of these spin magnetic moments point in one direction as shown in Fig. 1.9.

Hence even in the absence of external field, the magnetic moments align themselves parallel to each other and give rise to magnetic field.

Effect of magnetic field

To these materials even if a small external magnetic field is applied, the magnetic moments which are already aligned parallel, reorient itself along the direction of the magnetic field and they become very strong magnets.

Properties

1. Since some magnetisation is already existing in these materials, all the magnetic lines of force passes through it (Fig. 1.10).

2. They have permanent dipole moment. So they act as strong magnets.

3. They exhibit magnetisation even in the absence of external field. This property is called Spontaneous magnetisation.

4. It's susceptibility is positive and high and it is given by

χ= C / T‒θ

5. When the temperature is greater than curie temperature, ferromagnetic material becomes paramagnetic material.

6. Permeability is very much greater than 1.

Examples: Ni, Co, Fe etc.,

COMPARISON CHART FOR DIA, PARA AND FERRO‒MAGNETIC MATERIALS

Dia‒magnetic material

1. In diamagnetic material there are equal number of electron spins which are randomly oriented and hence the net magnetic moment is zero.

2. When the external magnetic field is applied, the electrons will align perpendicular to the field direction and hence it reduces the magnetic induction present in the material. Thus they are name as weak magnets.

3. When the material is placed in the magnetic field, the magnetic flux lines are repelled away from the material.

4. The susceptibility negative ( χ = ‒ve).

5. The susceptibility is independent of temperature.

6. Permeability is less than 1.

7. When the temperature is less than critical temperature, the diamagnetism suddenly disappears and becomes a normal material.

8. Examples: Gold, antimony, bismuth, water, hydrogen, alcohol, germanium, silicon etc.

Para‒magnetic material

1. In paramagnetic material there are unequal number of electron spins and hence there exists a permanent magnetic moment.

2. When the external magnetic field is applied, the electrons will align parallel to the field direction and hence the material is magnetised. Thus they are named as strong magnets.

3. When the material is placed in the magnetic field, the magnetic flux lines passes through the material.

4. The susceptibility is positive and small (χ =+ve)

5. The susceptibility varies inversely with the absolute temperature.

6. Permeability is greater than 1.

7. When the temperature of the material is less than Curie temperature, para‒magnetic material converted into diamagnetic material.

8. Examples: Platinum, chromium, aluminium, copper sulphate, manganese sulphate etc.

Ferro‒magnetic material

1. In ferromagnetic material there will be large number of unequal electron spins and hence there exists enormous amount of permanent magnetic moment.

2. When the external magnetic field is applied, the electrons which are already aligned parallel will reorient itself along the field direction and will be very easily magnetised. Thus they are named as very strong magnets.

3. When the material is placed in the magnetic field, the magnetic flux lines are highly attracted towards the centre of the material.

4. The susceptibility is positive and large (χ=+ve)

5. The susceptibility depends upon the temperature.

6. Permeability is very much greater than 1.

7. When the temperature is greater than Curie temperature, the ferromagnetic material is converted into paramagnetic material,

8. Examples: Iron, nickel, cobalt, steel, etc.

4. FERRIMAGNETISM AND FERRITES

It is a special case of magnetic material and it is composed of two sets of different transition metal ions having different values of magnetic moment with antiparallel alignment as shown in Fig. 1.12.

Hence these material have antiparallel magnetic moments of different magnitudes, giving rise to fairly large magnetic moment in the presence of external magnetic field (Fig. 1.13).

Properties

1. The susceptibility is very large and is positive represented by

 χ = C  / T±0

when T> T_N

2. Beyond the Neel temperature, χ decreases.

3. These materials have low eddy current losses, and low hysteresis losses.

4. They have hysteresis loop in the form of a square and hence will have low coercivity.