See Article History Adiabatic demagnetization, process by which the removal of a magnetic field from certain materials serves to lower their temperature. This procedure, proposed by chemists Peter Debye and William Francis Giauque independently, , provides a means for cooling an already cold material at about 1 K to a small fraction of 1 K. The mechanism involves a material in which some aspect of disorder of its constituent particles exists at 4 K or below liquid helium temperatures. Magnetic dipoles—i.
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See Article History Adiabatic demagnetization, process by which the removal of a magnetic field from certain materials serves to lower their temperature. This procedure, proposed by chemists Peter Debye and William Francis Giauque independently, , provides a means for cooling an already cold material at about 1 K to a small fraction of 1 K. The mechanism involves a material in which some aspect of disorder of its constituent particles exists at 4 K or below liquid helium temperatures.
Magnetic dipoles—i. Under these conditions the dipoles occupy these levels equally, corresponding to being randomly oriented in space. When a magnetic field is applied, these levels become separated sharply; i. If the magnetic field is applied while the paramagnetic salt is in contact with the liquid helium bath an isothermal process in which a constant temperature is maintained , many more dipoles will become aligned, with a resultant transfer of thermal energy to the bath. If the magnetic field is decreased after contact with the bath has been removed, no heat can flow back in an adiabatic process , and the sample will cool.
Such cooling corresponds to the dipoles remaining trapped in the lower energy states i. Temperatures from 0. Much lower temperatures can be attained by an analogous means called adiabatic nuclear demagnetization. This process relies on ordering aligning nuclear dipoles arising from nuclear spins , which are at least 1, times smaller than those of atoms.
With this process, temperatures of the ordered nuclei as low as 16 microdegrees 0. Learn More in these related Britannica articles:.
The Adiabatic Demagnetization Refrigerator (ADR):
John Weisend, was originally published in the Spring issue of Cold Facts as part of his series, Defining Cryogenics. Reaching temperatures below 1K requires different techniques than the various helium gas cycles found in large scale refrigeration plants and small cryocoolers. This technique takes advantage of the fact that the entropy of paramagnetic materials in a magnetic field is lower than when no field is present. The lower entropy comes from the magnetic regions in the paramagnetic material being aligned and thus more ordered in the presence of a magnetic field. A more ordered solid has lower entropy.
Adiabatic Demagnetization Refrigeration
Current research has been used to describe alloys with a significant magnetocaloric effect in terms of a thermodynamic system. Such materials need to show significant temperature changes under a field of two tesla or less, so that permanent magnets can be used for the production of the magnetic field. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms. In a paramagnetic salt ADR, the heat sink is usually provided by a pumped 4 He about 1. The minimum temperature attainable is determined by the self-magnetization tendencies of the refrigerant salt, but temperatures from 1 to mK are accessible. Dilution refrigerators had for many years supplanted paramagnetic salt ADRs, but interest in space-based and simple to use lab-ADRs has remained, due to the complexity and unreliability of the dilution refrigerator.
GSFC Adiabatic Demagnetization Refrigerator
No matter when we put anything warm inside, the refrigerator will immediately start cooling it down. Likewise, any heat that leaks in through the insulation goes right back out. But into the room it goes, because energy cannot be destroyed. The ADR does not run continuously. It stores the heat that it absorbs, both heat from cooling warm objects and heat that leaks in. The part of the ADR that stores the heat is called the "salt pill". Often, the material is one of the general class of materials called "salts", which includes table salt as well as many other chemicals.