What is Adiabatic Demagnetization?
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.
Here, the removal of a magnetic field is applied to a magnetic substance when the latter has been thermally isolated from its surroundings. This process concerns paramagnetic substances almost exclusively. In which case, a drop in temperature of the working substance is produced known as magnetic cooling
What are the Precautions that need to be taken While Handling Magnets?
While working with magnets, we should not do the following:
We should not bring any magnet near any magnetic appliance that works with electricity
Magnets tend to lose their properties when heated, beaten or dropped from a height
They should be separated by a piece of wood and two pieces of soft iron should be present at their ends
Why would You want to Demagnetize a Magnet?
You may be wondering as to why you would want to ruin a perfectly good magnet. The answer is simple: sometimes magnetization is not desirable. For example, if you have a magnetic tape drive or other data storage device and wish to dispose of it, you don’t want just anyone to be able to access the data. Demagnetization is one of the ways to remove the data and improve security.
There are various situations in which metallic objects become magnetic and create problems. In some cases, the problem is that the metal will attract other metals towards it, while in a few other cases, the magnetic field itself presents issues. Examples of materials that are demagnetized include flatware, engine components, tools, metal parts following machining or welding and metal moulds.
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.
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 procedures.
Nuclear adiabatic demagnetization experiment helps attain much lower temperature. This technique depends on the nuclear dipoles' alignment, which is around 1000 times smaller 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 the 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.
FAQs on Adiabatic Demagnetization
1. What is adiabatic demagnetization?
Adiabatic demagnetization is a scientific technique used to achieve ultra-low temperatures, often approaching absolute zero (as low as 1K or even lower). The process involves applying and then removing a strong magnetic field from a specific type of material, typically a paramagnetic salt, while it is thermally isolated from its surroundings. This manipulation of the magnetic field causes a significant drop in the material's temperature.
2. What is the fundamental principle behind cooling by adiabatic demagnetization?
The core principle relies on the relationship between a material's entropy (a measure of disorder) and its magnetic state. The process works in two main stages:
- Isothermal Magnetization: When a strong magnetic field is applied, the magnetic dipoles within the paramagnetic salt align themselves with the field. This alignment creates a more ordered, low-entropy state and releases heat, which is transferred to a cold reservoir (like liquid helium).
- Adiabatic Demagnetization: After being thermally isolated, the magnetic field is removed. The dipoles tend to return to their natural, disordered, high-entropy state. To do this, they require energy, which they draw from the substance's own internal thermal energy (lattice vibrations). Since no external heat can enter, this absorption of internal energy causes the substance to cool down dramatically.
3. What are the key steps involved in the adiabatic demagnetization process?
The process to achieve magnetic cooling follows a precise sequence:
- The sample, typically a paramagnetic salt, is placed in contact with a cold reservoir (e.g., liquid helium) to cool it to an initial low temperature.
- A strong magnetic field is applied, causing the magnetic dipoles in the salt to align. This process, called isothermal magnetization, generates heat that is absorbed by the cold reservoir.
- The sample is then thermally isolated from the reservoir, ensuring no heat can enter or leave.
- The magnetic field is slowly reduced to zero. This is the adiabatic demagnetization step. With the field gone, the dipoles randomise, absorbing thermal energy from the sample itself and causing its temperature to drop significantly.
4. Which types of materials are most suitable for the adiabatic demagnetization process?
Materials suitable for adiabatic demagnetization must possess magnetic dipoles that are randomly oriented at low temperatures. The most common materials are paramagnetic salts, such as gadolinium sulfate or cerous magnesium nitrate. For reaching even lower temperatures, scientists use nuclear paramagnets, which are substances whose atomic nuclei have magnetic moments. These are used because the interactions between nuclear moments are much weaker than those between atomic moments, allowing for cooling to microkelvin temperatures.
5. Why exactly does removing the magnetic field cause the temperature to drop?
Removing the magnetic field causes a temperature drop because the process is performed adiabatically, meaning with no heat exchange with the outside world. When the external field is removed, the aligned magnetic dipoles are no longer held in an ordered state. They naturally move towards a state of maximum disorder (high entropy). The energy required for this re-randomisation must come from somewhere, and since no external heat is available, it is taken from the internal thermal energy of the substance itself. This draining of internal energy results in a profound cooling effect.
6. How does an Adiabatic Demagnetization Refrigerator (ADR) differ from a standard household refrigerator?
An ADR and a household refrigerator operate on entirely different principles and scales:
- Working Principle: An ADR uses the magnetocaloric effect, manipulating a material's temperature with a magnetic field. A standard refrigerator uses a mechanical cycle of compressing and expanding a refrigerant gas.
- Working Substance: An ADR uses a solid paramagnetic salt as its cooling agent. A standard refrigerator uses a fluid refrigerant (like a hydrofluorocarbon).
- Temperature Range: ADRs are designed to achieve extremely low temperatures, typically from around 4K down to millikelvins (0.001 K) or even lower. A household refrigerator only cools to about 2-4°C (275 K).
7. What is the main difference between electronic and nuclear adiabatic demagnetization?
The primary difference lies in the source of the magnetic moments being manipulated:
- Electronic demagnetization uses the magnetic moments arising from the electrons in the atoms of a paramagnetic material. It is effective for cooling substances down to the millikelvin (10-3 K) range.
- Nuclear demagnetization uses the magnetic moments of the atomic nuclei themselves. These moments are about a thousand times weaker than electronic moments. Because the interactions are weaker, this method can overcome the limitations of the electronic method and achieve far lower temperatures, into the microkelvin (10-6 K) or even picokelvin (10-12 K) range.
8. What are the ultimate limitations on the low temperatures achievable with adiabatic demagnetization?
The cooling process is not infinite. The lowest achievable temperature is limited by the very small, residual internal magnetic fields and interactions that exist within the paramagnetic substance itself. Once the sample's temperature becomes so low that its thermal energy is comparable to the energy of these internal interactions, the magnetic dipoles will spontaneously align. At this point, removing the external field no longer produces a significant randomisation or further cooling. This fundamental limit is why nuclear paramagnets, with their much weaker internal interactions, can be used to reach lower temperatures than electronic paramagnets.
