What is Magnetic Susceptibility - Formula, Definition, Unit, FAQs

Magnetic susceptibility gives a measure of the response of the material to an external magnetic field indicating whether it is attracted by the field or repelled. This property is central to defining the magnetic behaviour of substances, i.e., whether they are diamagnetic, paramagnetic, or ferromagnetic. The real importance of magnetic susceptibility manifests in modern technologies such as MRI machines for medical imaging, magnetic storage devices, and even in geophysical aspects of demonstrating the magnetic attributes of the Earth along with mineral explorations.

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Magnetic Susceptibility Definition?

The magnetic susceptibility of a magnetic substance is defined as the ratio of the intensity of magnetization to the magnetic intensity.

Magnetic Susceptibility Formula

The magnetic Susceptibility Formula is given by, $X_m$

$$
\chi_m=\frac{I}{H}
$$

Where,

I is the intensity of magnetization

H is magnetic intensity

The magnetic susceptibility of a magnetic substance gives the measure of its aptness to acquire magnetism. As magnetic susceptibility is the ratio of the two quantities having the same units $\left(A m^{-1}\right)$ it has no units.

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Magnetic Materials

On the basis of the magnetic behaviour of different magnetic materials, Faraday divided the magnetic materials into three classes:-

1. Paramagnetic:- The substances, which are placed in a magnetic field are feebly magnetized in the direction of the magnetizing field, and are called paramagnetic substances.

When a paramagnetic substance is placed inside an external magnetic field, the magnetic field inside the paramagnetic field is found to be slightly greater than the external magnetic field. A paramagnetic substance tends to move from the weaker part of the magnetic field to the stronger part when placed in a non-uniform magnetic field. The paramagnetic effects are perceptible only when a strong magnetic field is there. Some of the few examples of paramagnetic substances are aluminium, sodium, antimony, platinum, copper chloride, liquid oxygen etc.

The magnetic susceptibility $\chi_{m!}$ of paramagnetic substances has a small positive value. Since, for a paramagnetic substance, ' l ' has a small positive value, from the relation $\chi_m=\frac{I}{H}$, it follows that $\chi_m$ will have a small positive value. It is of the order of $10^{-5}$ to $10^{-3}$.

The susceptibility of paramagnetic substances is inversely proportional to their absolute temperature.

2. Diamagnetic:- The substances, which are placed in a magnetic field are feebly magnetized in a direction opposite to that of the magnetizing field, and are called diamagnetic substances.

When we place a diamagnetic substance inside an external magnetic field, the magnetic field inside the diamagnetic field is found to be slightly less than the external magnetic field. It is also noticed that if a diamagnetic sample is placed inside a non-uniform magnetic field, then it tends to move from the stronger part to the weaker part of the magnetic field. It may be pointed out that the diamagnetic effects are too feeble to be detected unless the applied magnetic field is strong. Some of the few examples of diamagnetic substances are copper, zinc, bismuth, silver, gold, lead, glass, marble, water, helium, etc.

The magnetic susceptibility $\chi_m$ of a diamagnetic substance has a small negative value. Since, for a diamagnetic substance, I' has a small negative value, from the relation $\chi_m=\frac{I}{H}$ It follows that $\chi_m$ will have a small negative value. It is of the order of $10^{-6}$ to $10^{-3}$ (negative).

The susceptibility of a diamagnetic substance does not change with temperature for practical purposes. However, bismuth at low temperatures is an exception.

3. Ferromagnetic:- Those substances, when placed in a magnetic field are strongly magnetized in the direction of the magnetizing field, are called ferromagnetic substances.

When a ferromagnetic substance is placed inside an external magnetic field, the magnetic field inside the ferromagnetic field is found to be greatly enhanced than the external magnetic field. As a result, when a ferromagnetic substance is placed in a non-uniform magnetic field, it quickly moves from the weaker part to the stronger part of the magnetic field. In other words, the ferromagnetic effects are perceptible even in the presence of a weak magnetic field. Some of the few examples of ferromagnetic substances are iron, nickel, cobalt, alnico etc.

The magnetic susceptibility $\chi_m$ of ferromagnetic substances has a large positive value. It follows from the relation $\chi_m=\frac{I}{H}$. It is of the order of several thousand. The susceptibility of ferromagnetic substances decreases with the rise in temperature.