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Magnetic Properties of Materials

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Magnetic Properties of Materials - Lesson Summary

Based on magnetic susceptibility and relative magnetic permeability, materials are classified into three categories: diamagnetic, paramagnetic and ferromagnetic materials.

Diamagnetic materials are those that are repelled by magnets. Magnetic susceptibility �� and the relative magnetic permeability ��r of the diamagnetic substances are given below.

−1 ≤ �� < 0, 0 ≤ ��r < 1.

Superconductors exhibit perfect diamagnetism. When a material is cooled below its transition temperature, it becomes a superconductor. In superconducting state, the material behaves as a perfect conductor of electricity and a perfect diamagnetic material.

When a material makes a transition from the normal state to the superconducting state, it actively excludes the magnetic field from its interior, and hence repels magnets. This is called Meissner effect.

Paramagnetic materials those that are weekly attracted by magnets. Magnetic susceptibility of paramagnetic materials is small and positive. Relative magnetic permeability of these materials is slightly greater than one. The magnetisation of the paramagnetic material is inversely proportional to its absolute temperature. Thus, the magnetic susceptibility, ��, depends on the nature and the temperature of the material.

Ferromagnetic materials are those that are strongly attracted by the magnets. The field lines are highly concentrated inside the ferromagnetic material. When a bar of ferromagnetic material is placed in an external magnetic field of induction B0 the magnetisation of the domains orient in the direction of the applied magnetic field. When the external magnetic field is removed, some ferromagnetic materials can retain their magnetisation, and such materials are called hard ferromagnets or hard magnetic materials. Materials, which can lose their magnetisation, when the external magnetic field is removed are called soft ferromagnetic materials.

The magnetic susceptibility of ferromagnetic materials, ≫ +1. The relative magnetic permeability of these materials, ��r ≫ +1. The temperature beyond which a ferromagnetic material becomes a paramagnetic material is called Curie Temperature.

For a given value of magnetic intensity (H), the magnetic field (B) depends on the previous history of the sample. This phenomenon is called hysteresis.

Ferromagnets stay magnetised after being subjected to an external magnetic field, and this tendency to retain magnetism even after the external magnetic field is removed is called retentivity.

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