Magnetic Properties of Material

Classification of Magnetic Materials

On the basis of their magnetic properties different materials are classified as:

• Diamagnetic substance

• Paramagnetic substance

• Ferromagnetic substance

Diamagnetic Substance

Michael Faraday discovered that a specimen of bismuth was repelled by a strong magnet. Diamagnetism occurs in all materials. These materials are those in which individual atoms do not possess any net magnetic moment. [Their orbital and spin magnetic moment add vectorially to become zero]. The atoms of such material however acquire an induced dipole moments when they are placed in an external magnetic field.

The diamagnetic materials are Type 1 superconductors as they exhibit perfect conductivity and perfect diamagnetization when cooled to very low temperature. The superconductor repels a magnet and in turn is repelled. Such perfect diamagnetism in superconductors exhibiting the above phenomena is called Meissner effect.

Some important properties are:

• When suspended in a uniform magnetic field they set their longest axis at right angles to the field as shown

2) In a non-uniform magnetic material, these substances move from stronger parts of the field to the weaker parts. For e.g.,. when diamagnetic liquid is put in a watch glass placed on the two pole pieces of an electromagnet and current is switched on the liquid accumulates on the sides.

[Note on increasing the distance between the pole, the effect is reversed]

3) A diamagnetic liquid in a U shaped tube is depressed, when subjected to a magnetic field.

4) The lines of force do not prefer to pass through the specimen, since the ability of a material to permit the passage of magnetic lines of force through it is less.

5) The permeability of the substance, that is, m_r < 1.

6) The substance loses its magnetization as soon as the magnetizing field is removed.

7) Such specimen cannot be easily magnetized and so their susceptibility is negative.

Example: Bismuth, antimony, copper, gold, quartz, mercury, water, alcohol, air, hydrogen etc.

Paramagnetic Substance

Paramagnetic substance are attracted by a magnet very feebly. In a sample of a paramagnetic material, the atomic dipole moments initially are randomly oriented in space.

When an external field is applied, the dipoles rotate into alignment with field as shown

The vector sum of the individual dipole moments is no longer zero.

Some important properties are:

• The paramagnetic substance develops a weak magnetization in the direction of the field.

• When a paramagnetic rod is suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field.

• The lines of force prefer to pass through the material rather than air that is m_r > 1 that is their permeability is greater than one.

• As soon as the magnetizing field is removed the paramagnetics lose their magnetization.

• In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).

• A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.

• The magnetization of paramagnetism decreases with increase in temperature. This is because the thermal motion of the atoms tend to disturb the alignment of the dipoles.

Example:

Aluminum, platinum, chromium, manganese, copper sulphate, oxygen etc.,

Ferromagnetic Substance

Ferromagnetism, like paramagnetism, occurs in materials in which atoms have permanent magnetic dipole moments. The strong interaction between neighboring atomic dipole moments keeps them aligned even when the external magnetic field is removed.

Some important properties are:

• These substances get strongly magnetized in the direction of field.

• The lines of force prefer to pass through the material rather than air that is m_r>1 that is their permeability is greater than one.

• In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).

• A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.

• For ferromagnetic materials m_r is very large and so its susceptibility i.e., X_m is positive.

• Ferromagnetic substances retain their magnetism even after the magnetizing field is removed.

• The effectiveness of coupling between the neighboring atoms that causes ferromagnetism decreases by increasing the temperature of the substance. The temperature at which a ferromagnetic material becomes paramagnetic is called its curie temperature. For example the curie temperature of iron is 1418^oF, which means above this temperature, iron is paramagnetic.

Example: Iron, cobalt, nickel and number of alloys.

Curie Law in Magnetism

Pierre Curie experimentally discovered that intensity of magnetization I is directly proportional to B_o (flux density in Vacuum) and inversely proportional to the absolute temperature.

T of the material

That is I a B_o/T

or I=c B_o/T

Here, C is Curies constant.

The saturation region explains that at a particular stage, all atomic dipoles present in the specimen align in the direction of the field.

Domain Theory

With ferromagnetic substance, there are regions roughly 0.01 mm in size in which coupling of dipoles is perfect. Such regions are called domains. In each domain, however, the dipoles point in different directions and so add up vectorially to give zero in an unmagnetized ferromagnet as shown below.

(a)

On placing ferromagnets in an external magnetic field, the domains having magnetic moments in the direction of magnetic field start growing in size at the cost of other domains.

(b)

Thus, the number of magnetic moments pointing in the direction of the magnetic field increases and for a strong field, the material gets strongly magnetized.

Hysteresis

Consider an iron being magnetized slowly by a changing magnetizing field (H). The intensity of magnetization is found to increase along OA. On decreasing H slowly, I also decreases but does not follow AO. When H = 0, I has a non-zero valve equal to OB. This implies that some magnetism is left in the specimen. This value of I which is non-zero when H = 0 that is OB is called retentivity or residual magnetism.

When the field is applied in the reverse direction, the I decreases along BC till its zero at C. The valve of H which has to be applied to the magnetic material in reverse direction so as to reduce its residual magnetism to zero, is called its coercivity. On increasing H further, I increases along CD till it acquires a saturation at D. On changing the field, I follows a path DEFA. This closed loop is called hysteresis loop and represents cycles of magnetization a specimen has undergone. The hysteresis therefore refers to lagging behind. Here I lags behind H.

The shape and size of hysteresis loop is characteristic of each material, because of their difference in retentivity, coercivity etc.