Adiabatic Demagnetization

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What is Adiabatic Demagnetization?

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Reaching low temperatures involves numerous techniques, and one such technique refers to the Adiabatic Demagnetization. This method works with the heat properties and magnetic properties of some molecules. Adiabatic Demagnetization Refrigerator is useful to cool substances at 5K to about 1K. Go through this article and acknowledge what materials can be cooled with the help of ADR. Also, understand the process of cooling due to adiabatic demagnetization. 

Adiabatic Demagnetization: Explain What is it?

Magnetic cooling is one of the efficient methods of cooling objects. It capitalizes on the relationship between the applied magnetic field effects and the entropy of a material. Adiabatic demagnetization comes under magnetic cooling, exploiting paramagnetic properties to cool some materials down. It is based on the fact that the entropy of paramagnetic materials is lower in the magnetic field. The magnetic regions aligned in the paramagnetic field originate lower entropy. Thus, randomness is less in the presence of a magnetic field, and hence substance can reach a temperature below one Kelvin.

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The above picture shows the effect of a magnetic field on a paramagnetic substance when placed in a cold reservoir to cool down. 

Cooling by Adiabatic Demagnetization Process

  • The sample, which has to be cooled, is permissible to touch a cold reservoir. This cold reservoir is maintained at a constant temperature of around 2-3 K. A magnetic field is induced in the sample region. 

  • The magnetic field strength is increased when the sample comes in thermal equilibrium with the cold reservoir. The particles get aligned with the magnetic field, and hence, the system becomes well-ordered. It causes a decrease in the entropy of the sample. However, the sample's temperature is the same at this point as that of the cold reservoir. It refers to adiabatic magnetization.

  • The sample taken is now isolated from the cold reservoir, and the strength of the magnetic field is lessened. There is no change in the randomness of the sample salt. However, there is a decrease in the sample salt's temperature due to a reduction in the strength of the magnetic field. If the sample was already at a low temperature, this temperature reduces to a greater extent. 

The adiabatic demagnetization process can be repeated by permitting sample salt to come at low temperatures. 

Adiabatic Demagnetization Process for Nuclear Paramagnets

An adiabatic demagnetization process is useful to obtain extremely low temperatures. For the electronic paramagnetic salts, this method is useful to attain low temperatures of about 1K. However, for the nuclear paramagnets, the temperature can be decreased as far as possible. 

Nuclear paramagnets are several compounds or elements that contain zero magnetic moments; however, their nuclei carry some nuclear magnetic moments. These magnetic moments are useful for magnetic refrigeration. Nuclear demagnetization refrigerator was proposed for this technique. Nowadays, this refrigeration process is used to avoid some disadvantages of electronic paramagnetic refrigeration procedure. 

Nuclear adiabatic demagnetization experiment helps attain much low temperature. This technique depends on the nuclear dipoles' alignment, which is around 1000 times minor compared to atoms. With this method's help, the temperature of the ordered nuclei can be reduced to 0.000016 degrees. 

Principle of Adiabatic Demagnetization

The principle of the adiabatic demagnetization process is applicable to magneto-caloric materials. The principle follows that when these materials are placed in a magnetic field, they start heating up. However, when removed from the magnetic field, then they cool down. The principle of adiabatic demagnetization of paramagnetic salts is as follows:

  • Each atom of the paramagnetic salt is considered a tiny magnet. When there is no magnetic field, then all atoms of the salt get oriented randomly. As a result, the total magnetic force is zero. However, after coming in contact with the strong magnetic field, atoms of the salt align themselves to the magnetic field direction. In this process, there is a rise in temperature. 

  • On demagnetization, that is, removing magnetic field, atoms of paramagnetic salts come back to the random orientation. It results in a reduction of temperature as the atoms do the work. Moreover, this procedure occurs adiabatically. As per the Second Law of Thermodynamics, there will be a change in the work done and hence temperature changes. 

Final Thoughts

Adiabatic demagnetization experiment is a useful method to cool down certain substances' temperatures by placing them in a magnetic field. It is an effective magnetic cooling method to cool down materials, usually in gaseous form. The method works on the fact that the entropy of the paramagnetic substance, when placed in a magnetic field is zero. Apart from the electronic paramagnets, adiabatic demagnetization is useful to lower the temperature of nuclear paramagnets. 

FAQ (Frequently Asked Questions)

1. Which Type of Materials is Suitable for Adiabatic Demagnetization?

The mechanism of demagnetization involves a material in which its constituent particles disorder exists at 5K or below it. The magnetic dipoles present in a paramagnetic salt crystal consists of this disorder property. The dipoles are arbitrarily oriented in these salts. On applying a magnetic field, the dipoles' levels become separated abruptly. When the material comes in contact with the liquid helium bath, more dipoles become aligned. As a result, there is a transference of thermal energy from the paramagnetic salt to the helium bath; removing the salt from the helium bath and decreasing the magnetic field results in no flow back of heat. As a result, the salt taken cools down. 

2. Is there Any Limitation of the ADR Procedure?

Adiabatic demagnetization experiment is a useful technique to cool an element or compound to very low temperatures. However, there are some limitations of this technique, which cannot be avoided. For instance, if the sample salt is an electronic paramagnet, then these can attain the lowest temperatures on the order of 1 millikelvin using demagnetization. However, if the sample taken is a nuclear paramagnet, then the achievable temperatures are quite much lower. It is because the interactions in the case of nuclear paramagnet are much weaker compared to electronic paramagnet. Nuclear adiabatic demagnetization is observed to cool down the temperature to about 250 Picokelvins and may cool them further.