Curie's Law
According to the Curie’s Law, the magnetization which is present in a paramagnetic material is said to be directly proportional to the applied field of magnetic. If the object which we have used is heated then the magnetization is viewed to be temperature which is inversely proportional. The law which we are discussing was discovered by the French physicist named Pierre Curie.
Most of the elements and along with some compounds which are paramagnetic in nature. Paramagnetism is exhibited by compounds which are containing palladium, iron, platinum, and the earth rare elements. In such compounds which are made up of atoms of these elements have some inner shell electrons that are incomplete. This causes their unpaired electrons to spin like orbits and tops like satellites. This makes the atoms magnetic which tend to align with and strengthen an applied field of magnetic.
Faraday became the first to direct substances into being diamagnetic or paramagnetic. He based this classification accordingly. His observation of the force is mainly how it is exerted on the substances of an inhomogeneous magnetic field. At very moderate field strengths, the magnetization of M of a substance is very linearly proportional to the strength of the applied field H.
The magnetization is clearly specified by the magnetic susceptibility χ, which is always defined by the relation M = χH. A sample of volume V which is placed in a field H is directed in the x-direction and is increasing in that direction at a rate of dH/dx will experience a force in the x direction of F = i.e χμ0VH (dH/dx). The magnetic susceptibility of χ is positive, the force will also be in the direction of increasing field strength, only if χ is negative, it will be in the direction of decreasing field strength.
Measurement of the force F in a known field H with a known gradient dH/dx is the basis of the number of specified methods for determining χ. In matters as concerned with free magnetic dipole moments, the determination of these moments is normally random and, as a consequence, the substance has no specific net magnetization.
When a magnetic field is applied, the dipoles will no longer be deeply oriented; more dipoles will always point with the field than against the field. When this results in a very net positive magnetization in the direction of the field, the substance has a positive susceptibility and is also classified as being paramagnetic.
There is a third category of matter in which moments are not normally present but appear to be under the influence of an external magnetic field. The intrinsic moments of conductive electrons in metals always end up behaving this way. We always end up finding a small positive susceptibility which remains independent of the temperature compatible with diamagnetic contribution, so that the overall susceptibility of a metal can be determined as positive or negative.
Magnetic effect of strongly paramagnetic substances decreases with temperature increase because of the dealignment produced by the greater random motion of the atomic magnets. Magnets exhibiting weak paramagnetism are independent of temperature which is found in many elements which are metallic in the solid state. For example, sodium and other metal alkali. It is because an applied magnetic field affects the spin of some of the loosely bound conduction electrons. The susceptibility value that is a measure of the relative amount of induced magnetism for magnets that are paramagnetic materials is always positive and at room temperature. That is, we can say typically about 1/100,000 to 1/10,000 for the magnets which are weakly paramagnetic and about 1/10,000 to 1/100 for strongly paramagnetic magnets.
Curie's Law Formula
Curie’s Law can be framed very easily into an equation.
That is - M = C x (B/T)
Wherein,
M is = Magnetism
B is = Magnetic field in Tesla
T is = absolute temperature in Kelvins
And C is = Curie constant
Curie Temperature
Paramagnetic materials such as platinum or aluminum sometimes are magnetized in a magnetic field and their magnetism becomes extinct when the field doesn’t exist. Ferromagnetic materials like iron and nickel that retain their properties of magnetic fields when the field is erased.
The temperature of Curie is the one at which ferromagnetic material turns to paramagnetic on heating. This kind of transition which we are observing over here is used in optical storage media for erasing and inserting the data which is new.
In a material which is paramagnetic the magnetization of the material is said to be directly proportional to an applied field of the magnet. However, if the material is heated then this proportionality is reduced basically for a fixed value of the magnetic field which is inversely proportional to T that is the temperature.
M = C.B/T
Paramagnetism and Diamagnetism
The magnet which is paramagnetic in nature exhibits a kind of magnetism where several objects are attracted to them through an externally applied magnetic field. On the other hand, the materials which are diamagnetic are repelled by magnetic fields and develop an induced field of magnetic direction which is said to be opposite to that of the applied magnetic field.
These materials which we have discussed include most of the elements which are chemical elements and some compounds which have a magnetic permeability greater than or equal to 1.
Magnetic Moment
The moment of a magnet which is induced by the applied field is linear to the weakness or the strength of the field. It is usually said that it needs a sensitive balance that is analytical to detect the different modern measurements and effect on the material which is paramagnetic. This is often conducted with a SQUID magnetometer.
Ferromagnetism - It is the mechanism by which certain materials form permanent magnetism. Ferromagnetism is the reason behind magnetism in magnets. Substances react weakly to the three other types of magnetism- paramagnetism, diamagnetism and antiferromagnetism. They can only be detected in laboratories by application of various instruments. Permanent magnets are ferromagnetic. They can be ferromagnetic too. Ferromagnetic materials can be divided into two : magnetically soft materials which don’t tend to stay magnetized for long and magnetically hard materials which stay magnetized for long. Permanent magnets are made from these hard ferromagnetic materials. Ferrimagnetic materials undergo a special processing in a strong magnetic field to develop very strong magnetic properties, making them hard to demagnetise.
Magnetic Susceptibility- A material will become magnetized in an applied magnetic field. It is a measure of how much it will do so. It refers to whether a material will remain attracted into, or be repelled out in a magnetic field, thereby giving way to paramagnetism or diamagnetism. Magnetization develops from atomic level magnetic properties of the particles from which they are made. The magnetic susceptibility can be measured by applying the macroscopic form of Maxwell’s equations.
Permeability- It is measured by the ability of a material to support the formation of a magnetic field within itself, or the ability of a material to become magnetized when exposed to an applied magnetic field. The permeability can also be known as the magnetic constant.
Curie Point- It is the temperature above which materials lose their magnetic properties. Certain materials go through a sharp change in their properties at this temperature.
Curie Constant- It is a property depending upon the material that relates its magnetic capability to its temperature. It is a material dependency property which relates the material’s magnetic susceptibility to its temperature through the application of Curie’s law.
Limitations of Curie’s Law
Curie Weiss Law fails to describe the capability of certain materials.
Points to remember about Curie’s Law :
Magnetic susceptibility is very inversely proportional to the absolute temperature.
The magnetisation of a paramagnetic material depends on and is directly proportional to the applied magnetic field.
Curie’s law is applicable only in certain materials.
Conclusion
The law of Curie and the temperature of Curie are important topics in IIT JEE. The few topics which are related to curie's law are the topics usually which fetch direct questions in the exam and so it becomes vital to master them. These topics like Curie's temperature and law include various formulae which fetch direct numerical questions. They are quite very easy and these topics don’t require much practice but it is very much important to clarify these concepts of curies.
FAQs on What is Curie's Law?
1. What is Curie's Law as explained in the NCERT syllabus?
Curie's Law states that the magnetisation (M) of a paramagnetic material is directly proportional to the applied magnetic field (B) and inversely proportional to the absolute temperature (T). In simple terms, a paramagnetic substance becomes less magnetic as it gets hotter, and more magnetic in a stronger external field.
2. What is the mathematical formula for Curie's Law?
The formula for Curie's Law is expressed as:
M = C * (B/T)
Where:
- M is the resulting magnetisation.
- C is the Curie constant, a material-specific property.
- B is the strength of the applied magnetic field (in Tesla).
- T is the absolute temperature (in Kelvins).
3. What is the Curie constant (C) and what factors does it depend on?
The Curie constant (C) is a characteristic property of a specific paramagnetic material. Its value depends on the intrinsic magnetic properties of the material's atoms or molecules, such as the density of magnetic dipoles within the material and the strength of each individual magnetic moment.
4. How does Curie's Law explain the behaviour of paramagnetic materials?
Curie's law explains the balance between two competing effects in paramagnetic materials. The external magnetic field (B) works to align the random atomic magnetic dipoles, causing magnetisation. Simultaneously, the thermal energy (related to Temperature T) causes random vibrations, which work to disalign these dipoles. The law shows that as temperature increases, the disaligning effect of thermal energy becomes stronger, thus reducing the overall magnetisation.
5. What is the Curie Temperature, and why is it important?
The Curie Temperature (Tc), or Curie Point, is a critical temperature unique to ferromagnetic materials. Above this temperature, a material loses its strong ferromagnetic properties and becomes paramagnetic. Its importance lies in marking a fundamental phase transition in the magnetic behaviour of matter, driven by thermal energy overcoming the internal aligning forces.
6. Does Curie's Law apply to ferromagnetic and diamagnetic materials?
No, Curie's Law does not apply to these materials. Here's why:
- Ferromagnetic Materials: Below their Curie Temperature, they exhibit spontaneous magnetisation and don't follow this law. Above the Curie Temperature, their behaviour is better described by the more complex Curie-Weiss Law.
- Diamagnetic Materials: Their weak magnetic opposition is largely independent of temperature, so Curie's Law is not applicable.
7. How does the Curie-Weiss Law differ from Curie's Law?
The Curie-Weiss Law is an adaptation of Curie's Law that describes the magnetic susceptibility of ferromagnetic materials above their Curie temperature. While Curie's Law is for ideal paramagnetic materials, the Curie-Weiss Law includes a term (the Weiss constant) to account for the interactions between the atomic dipoles, which are significant in ferromagnetic materials even in their paramagnetic phase.
8. Under what conditions does Curie's Law become invalid?
Curie's Law is an approximation that fails under certain extreme conditions. It becomes invalid in:
- Very strong magnetic fields: At high field strengths, magnetisation reaches a saturation point and is no longer directly proportional to the field.
- Very low temperatures: Near absolute zero, quantum mechanical effects and interactions between dipoles become significant, which are not accounted for in the simple law.
9. What is Pauli Paramagnetism and how is it different from the paramagnetism described by Curie's Law?
Pauli Paramagnetism is a weak form of paramagnetism observed in certain metals, arising from the alignment of conduction electron spins. The key difference is that Pauli paramagnetism is almost entirely independent of temperature, whereas the paramagnetism described by Curie's Law is strongly, and inversely, dependent on temperature.
10. What are some real-world examples or applications of the Curie Temperature?
A primary application of the Curie Temperature is in magneto-optical (MO) data storage. A laser heats a tiny spot on a disk above its Curie point, temporarily neutralising its magnetic properties. This allows a magnetic head to easily write or erase data. When the spot cools down, the new magnetic orientation is locked in. It is also a critical parameter in designing thermal switches and sensors.
