# Curie Weiss Law

## Curie Weiss Law

Curie-Weiss law is an important law in physics under the topic of electromagnetism. The Curi-Weiss law states that the magnetic susceptibility of a ferromagnetic material is greater than the Curie temperature point in the paramagnetic zone of the ferromagnet.

With this feature, the object's magnetic moment determines the torque of a magnet in response to the presence of an external magnetic field. In the magnetic substances above the Curie temperature, the moments are randomly oriented, determine the net magnetic polarization or magnetization equals to zero.

Examples of magnetic moments are the electric current loop, a bar magnet, an electron, or a molecule.

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The magnetization or magnetic polarization implies the density of induced or permanent magnetic moments of magnetic material in the field of the vector. The small electric current produced by the spin of electrons, the spin of nuclei, or mobility in an atom results in a magnetic moment that can form.

Net magnetization is determined by the external magnetic field in the response of the materials. Sometimes it can even exist in the lack of an external magnetic field. Examples of such materials are cold iron as spontaneous magnetization. There are some other materials with the corresponding qualities, like nickel and magnetite, and they are referred to as ferromagnets.

The temperature at which a ferromagnetic substance turns into ferromagnetic is called Curie temperature.

## What is mean by Curie?

Radioactivity is measured by the unit curie. It has a value of one curie (Ci) equal to 3.7 × 1010 decays/second. The point of temperature at which magnets change their magnetic characteristics is called Curie temperature. Curie temperature and Curie point are derived from Curie.

## Curie Weiss Law Formula

The expression or formula of Curie Weiss Law is formulated below:

X = C / T - Tc

where,

C is the Curie of specific material,

T is the absolute temperature,

Tc is the Curie temperature

The magnetic moments always depend on magnetic materials in the external fields. The magnetic moment is the relationship between the material of the magnetic field and the absolute temperature of the material.

## Limitations of Curie Weiss Law

Curie Weiss's law has many presumptions and inferences to calculate its susceptibility. Let us deal with the credibility of the Curie Weiss formula, which can be written as:

$$\mathrm{X\:\sim\:1/(T-Tc)y}$$

Where, T > Tc is true, but Curie Temperature Tc is replaced by the temperature, you will get a higher value than Curie Temperature Tc, which is false.

However, this is the widely used formula irrespective of the limitation.

## Weiss Law and Ferromagnetism

In the absence of an applied magnetic field magnetization arises in a substance of spontaneous magnetization, there experience Ferromagnetism. The most common example of ferromagnets is iron, cobalt, and nickel as well as a few alloys that have ferromagnetic features. Ferromagnetism occurs when the moments in molecules are aligned in the right order.

For ferromagnetism to be evident, ferromagnetic transition temperature (threshold temperature) should be reached; the temperature may be higher than 1000K for materials like such as iron, cobalt, Gd, and others. This ferromagnetic transition temperature happens in the absence of an external field when atomic magnetic dipoles point in the opposite direction.

For example, in iron, the magnetic moment is induced by the spinning of electrons in the outer shell of the nuclei. Pauli's exclusion principle says that two electrons will not spin in the same direction at the same place. This will cause repulsion unreasonable between the two electrons. Electrons spinning in opposite directions attract interactions with magnetic polarization. As a result of the attractive force present in electrons spinning in the opposite direction, atoms in the iron can align with each other. This may be formulated as follows:

The effective molecular field Hint, which relies on the magnitude of magnetization M in the influence of exchange forces;

Hint = M     --------------Equation (2)

The magnetization (M) can be written in another way as the sum and product of the magnetic susceptibility.

${\chi p\:\backslash s(H+\lambda M)=M}$      --------------- Equation (3)

The equation formula is the basic idea for the Curie-Weiss Law equation.

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## Curie Temperatures of Some Ferromagnetic Substances

Some Curie temperatures of ferromagnetic substances are:

• Curie temperature (Tc) of Iron (Fe) is 1,043K.

• Curie temperature (Tc) of Gadolinium (Gd) is 293K.

• Curie temperature (Tc) of Nickel (Ni) is 631K.

## Conclusion

The Curie-Weiss law is the important rule in electromagnetism, which is defined as, in the paramagnetic zone, the magnetic susceptibility is greater than the point of Curie temperature (Tc) of a ferromagnet. This temperature is the temperature of ferromagnetic compounds at the place of the paramagnetic field.

In the absence of an applied magnetic field, magnetization arises in a substance in the occurrence of spontaneous magnetization is called Ferromagnetism.

$${\chi p\:\backslash s(H+\lambda M)=M}$$

This formula is the basic step to deriving the Curie-Weiss Law equation.

## FAQs

Qns 1. What is the constant value of Curie?

Ans. The constant value of the Curie is 1.3047K

Qns 2. How can we calculate Curie?

Ans. The Curie can be calculated by dividing per second rate of decay by 3.7 x 1010 and the result is equal to 1 Curie.

Qns 3. What Is the Distinction Between Curie and Curie Weiss Temperatures?

Ans. The Curie temperature (Tc) is the temperature at which susceptibility of the material erupts.

$\mathrm{\chi = C/T - Tc}$ and $\mathrm{T \sim Tc}$

On the other hand, the Curie-Weiss temperature, holds for T >> T0 and is near to T0 for first-order transitions in Curie temperature.

Qns 4. What is the valid temperature range that matches to ferroelectric Curie-Weiss Law?

Ans. In ferroelectric materials, the temperature dependence of ε can be precisely shown by Curie- Weiss as follows:

$\mathrm{-\varepsilon = \varepsilon 0 +C/(T - T0)}$

where,

C is the Curie’s constant and T0 is the CW temperature; in general, it differs from Curie’s temperature (Tc)

${\varepsilon}$ becomes peak when it reaches T0

In ferroelectric materials, Tc= T0, and the phase transition is in second-order While Tc and T0 differ very largely in the first order

Qns 5. Which materials follow the Curie-Weiss law?

Ans. The materials that share ferromagnetic properties with iron, like Nickel and magnetite, are called Ferro magnets. The Curie temperature of materials varies, which is below the threshold temperature.

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