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Magnetic Materials

On the basis of mutual interactions or behavior of various materials in an external magnetic field, the materials are divided in three main categories.

Diamagnetic materials

Diamagnetism is the intrinsic property of every material and it is generated due to mutual interaction between the applied magnetic field and the orbital motion of electrons.

Paramagnetic materials

In these substances the inner orbits of atoms are incomplete. The electron spins are uncoupled, consequently on applying a magnetic field the magnetic moment generated due to spin motion align in the direction of magnetic field and induces magnetic moment in its direction due to which the material gets feebly magnetized. In these materials the electron number is odd (Fig. 21).
Fig. 21

Ferromagnetic materials

In some materials, the permanent atomic magnetic moments have strong tendency to align themselves even without any external field. These materials are called ferromagnetic materials.
In every unmagnetized ferromagnetic material, the atoms form domains inside the material. Different domains, however, have different directions of magnetic moment and hence the materials remain unmagnetized. On applying an external magnetic field, these domainsrotate and align in the direction of magnetic field (Fig. 22).
Fig. 22

Curie law

The magnetic susceptibility of paramagnetic substances is inversely proportional to its absolute temperature, i.e., 87702.png χ ∝ C/T; where C = Curie constant, T = absolute temperature.
On increasing temperature, the magnetic susceptibility of paramagnetic materials decreases and vice versa.
The magnetic susceptibility of ferromagnetic substances does not change according to Curie law.

Curie temperature (Tc)

The temperature above which a ferromagnetic material behaves like a paramagnetic material is defined as Curie temperature (Tc).
The minimum temperature at which a ferromagnetic substance is converted into paramagnetic substance is defined as Curie temperature. For various ferromagnetic materials its values are different, e.g., for Ni, TCNi = 358°C for Fe, TCFe = 770°C for CO, TCCo = 1120°C
At this temperature, the ferromagnetism of the substances suddenly vanishes.

Curie-Weiss law

At temperatures above Curie temperature, the magnetic susceptibility of ferromagnetic materials is inversely proportional to (T – Tc).
i.e. 87708.png
Here Tc = Curie temperature
χ-T curve is shown in Fig. 23 (for Curie-Weiss law)
Fig. 23

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