Cryocourse / Aalto University / Demagnetization refrigeration – J. Tuoriniemi . 1. Adiabatic Demagnetization. Refrigeration. – External. Abstract: A tiny adiabatic-demagnetization refrigerator (T-ADR) has been developed for a commercial superconducting quantum interference. An overview is given of the general principles and techniques used for the design and construction of compact, portable adiabatic demagnetization refrigerators.
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There is a need to have a reference entropy value for all substances for evaluating the efficiency of a reactive system.
Third law of thermodynamics provides a base value for the entropy. The third law of thermodynamics was formulated during the early part of twentieth century. The initial work was done primarily by W.
Nernst  and Max Planck . In the neighborhood of absolute zero, all ordinary methods of temperature measurement fail.
The fundamental features of all cooling process are that the lower the temperature achieved, the harder it is to go still lower.
Marching towards absolute zero temperature. Attaining low temperature Below 5 K is possible by Joule Kelvin expansion, by producing liquid helium.
Still lower temperature can be attained by adiabatic demagnetization of a paramagnetic salt. Temperature as low as 0.
Gadolinium sulphate is used here. Salt is hung by a fine nylon thread inside the salt tube such that it does not touch the sides. Next, the salt is exposed to a strong magnetic field of about Gauss.
Heat produced due to magnetization of the salt is transferred to the liquid helium without causing refrigerwtor increase in salt temperature. With the magnetic field still present, the inner chamber containing the salt is evacuated of gaseous helium.
Magnetic refrigeration – Wikipedia
Finally, the magnetic field is removed. The molecules disalign themselves, which require energy. This energy is obtained by the salt getting cooler in the process. Measurement of temperature In the neighborhood of absolute zero, all ordinary methods of temperature measurement fail.