Demagnetization paramagnetic salts

The paramagnetic salts useful for this type of thermometry are the same as that used in the ADR (adiabatic demagnetization refrigerator, see Section 7.3 and Fig. 7.7). The most commonly used salts are ... 

Magnetic thermometry has been developed chiefly to measure temperatures near absolute zero (below -458°F, or -272°C). These measurements are obtained by adiabatic demagnetization of a paramagnetic salt. Inductance can be measured with an AC bridge (as shown in Figure 3.164) whose balance is independent of frequency. The relationship between self-inductance and sus-... 

ADIABATIC COOLING - A method in which paramagnetic salts are precooled, and then demagnetized, thereby producing further cooling. 

A convenient way to maintain a detector at a low and constant temperature is to thermally couple the detector housing to a cryostat filled with liquid helium. At one bar the boiling point of helium is 4.2 K and less at lower pressures. For operation down to 0.3 K the isotope helium 3 can be used instead of the normally available helium, which is dominantly helium 4. Operation at even lower temperatures ( 0.1 K) is possible using demagnetization effects in certain paramagnetic salts ferric ammonium sulfate is an example (Castles, 1980 Serlemitsos et ai, 1990). 

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