Figure 3.7:
Adiabatic demagnetization. a) \(T-S\) diagram; b) scheme of the process steps.
Adiabatic demagnetization is the first method developed for cooling below 0.3 K. As illustrated in Fig. 3.7 a) it essentially contains two steps:
A-B: A sample of a paramagnetic salt (such as cerium magnesium nitrate), already cooled to low \(T\) by other means, is magnetized isothermally (contact with thermally well conducting He gas), which results in a decrease of the entropy for paramagnetic salt
B-C: adiabatic demagnetization (pumping out He gas to enforce thermal isolation) resulting in decrease of temperature
BUT: By this method (or by any other) \(T\) = 0 K cannot be reached since it would need an infinite number of steps, since the two curves must have a common point according to Nernst’s theorem of vanishing transition entropy (\(\Delta S = 0\) for \(T \rightarrow 0\)).
 |
 |
 |
 |
© J. Carstensen (TD Kin I)