27 December 2013

Different Types of Magnetic Materials and its Properties

Different Types:

1. Diamagnetic materials
          E.g: Cadmium, Copper, Silver, Bismuth, Tin, zinc, Gold, Niobium and its compounds.

2. Paramagnetic materials
          E.g: Aluminum, Calcium, Oxygen, Platinum, Titanium and Chromium.

3. Ferromagnetic materials
          E.g:
          a. Ferromagnetic materials
                   E.g: Iron, Cobalt, Nickel
          b. Anti-ferro magnetic materials
                   E.g: Ferrous oxide, Manganese oxide, Zinc ferrite
          c. Ferrimagnetic materials

                   E.g: Nickel ferrite, Manganese ferrite, Ferrous ferrite

Diamagnetic Materials

Definition
          These materials when placed in a magnetic field, becomes weakly magnetized in the direction opposite to that of the applied field. There is no permanent dipole moment in each atom. The induced magnetic moment produced in these materials during the application of the external magnetic field decreases the magnetic induction present in the specimen.

Origin
          A material contains a large number of electrons and the orbits of these electrons are randomly oriented in space. The current that is produced due to movement of electron in an orbit produces magnetic field in a direction at right angles to the plane of the orbit. This magnetic field induces a magnetic moment in the atom in a direction opposite to it. These magnetic moments are randomly oriented. Hence the magnetic moments of all such electron gets cancelled resulting in the net magnetism equal to zero in the material.

          When an external magnetic field is applied to the material, rotation of dipoles take place producing an induced dipole moment: This induced dipole moment opposes the applied field. The magnetism which is created in a direction opposite to that of the external field is called diamagnetism.

Characteristics of diamagnetic materials

1. Susceptibility ( m) of a diamagnetic material is always negative. The relative permeability μr < 1.
Example For Cadmium, ( m) = - 0.18 x10-6
For Copper, ( m) = - 0.086 x10-6
For Silver, ( m) = - 0.2 x10-6

2. When magnetic field is applied, it repels the magnetic lines of force. This property is exhibited by superconductors. Hence we call all superconducting materials (at low temperature) as perfect diamagnet. When the temperature is increased beyond it critical temperature, diamagnetism suddenly disappears and it behaves like a normal conducting material.

3. It does not depend on temperature and the strength of applied magnetic field.

4. No magnetic moment is present in the material.

Paramagnetic Materials

Definition
          Paramagnetic materials become weakly ionized when placed in a magnetic field in the same direction as that of the applied field. It has permanent dipole moment in each atom. When external magnetic field is applied, the induced magnetic moment is produced which increase the magnetic induction present in the specimen.

Origin
          The orientation of the magnetic moment along the direction of the external field gives rise to paramagnetism. The permanent magnetic moment arises due to orbital motion of electron around the nucleus and spin motion of electron about its own axis. The magnetic moment due to former disappears due to the effect of electric field of the neighbouring charges. But the magnetic moment due to electron spin are randomly oriented in the absence of external field. When the external field is applied, the magnetic moments tend to align in the direction of the applied field resulting in large magnetization. But due to the thermal agitation of the atoms the magnetic moments are partially aligned in the direction of the external field resulting in weak magnetization.

Characteristics

1. Susceptibility  ( m) is positive and small.

Example For aluminum, ( m) = 0.065 x10-6
     For Calcium, ( m) = 1.10 x10-6

The relative permeability μr > 1.

2. When magnetic field is applied to paramagnetic material, it is attracted towards the centre of the material.

3. Susceptibility is inversely proportional to absolute temperature of the material. m α (1/T)

Curie’s law for high temperature m = (C/T)

T = absolute temperature in Kelvin; C = Curie constant

At low temperature m = C/(T-θ)

θ – paramagnetic curie temperature

θ is always very low. When the temperature T < curie temperature, the paramagnetics becomes diamagnetic.

4. Spin alignment: All spins are randomly oriented.

Ferro Magnetic Materials

Definition
          Ferromagnetic materials are strongly magnetized in the direction of the applied magnetic field. It possesses enormous permanent magnetic moment in each atom. When external magnetic field is applied, a large amount of induced magnetic moment is produced which increases the magnetic induction present in the specimen.

Origin           
          The presence of permanent magnetic moments in the atoms or molecules in the specimen gives rise to ferromagnetism as this magnetic moment align themselves in the same direction as that of the external field. The exchange interaction between unpaired electrons of adjacent atoms in the crystal lattice gives rise to local molecular magnetic field resulting in spontaneous magnetization.

Characteristics

1. Magnetic susceptibility value is large and positive. The temperature dependence of susceptibility for ferromagnetic materials is said to be complex.

2. When magnetic field is applied to a ferromagnetic material, the magnetic lines of force are strongly attracted by the specimen.

3. Ferromagnetic materials exhibit hysteresis. Even if the magnetic field is removed from the material, it retains the magnetism due to spontaneous magnetization. They have permanent dipole moment.

4. The permeability of a ferromagnetic material is not a constant, as magnetic induction (B) does not vary linearly with magnetic field strength (H).

5. When the temperature of the ferromagnetic material is greater than its Curie temperature, then ferromagnetic is converted into a paramagnetic material.

Antiferromagnetism and Ferrimagnetism:

          The only type of magnetic order which has been considered thus far is ferromagnetism, in which, in the fully magnetized state, all the dipoles are aligned in exactly the same direction. There are, however, substances which show different types of magnetic order. In antiferromagnetic materials such as Cr and MnO, the dipoles have equal moments, but adjacent dipoles point in opposite directions. Thus the moments balance each other, resulting in a zero net magnetization. 

In ferromagnetic materials (also called ferrites) such as     MnFe2O4, the magnetic moments of adjacent ions are antiparallel and of unequal strength. So there is a finite net magnetization. By suitable choice of rare-earth ions in the ferrite lattices it is possible to design ferromagnetic substances with specific magnetizations for the use in electronic components.