Refrigeration by Adiabatic Demagnetization of a Paramagnetic Salt

Abstract

The year 1926 was a remarkable year for low-temperature physics and technology. H. Kamerlingh Onnes reached 0.7 K by pumping on his liquid ⁴He bath with an enormous battery of pumps. Realizing that there would be no element with a lower boiling point than helium, he predicted that the temperature of 0.7 K would remain the minimum temperature achievable by mankind in the laboratory unless somebody discovered a completely new refrigeration technology that did not depend on the latent heat of evapora­tion. (Of course, he was not aware that there exists a lighter helium isotope, ³He, which pushes this limit down by about a factor of three). In the same year this completely new refrigeration technology was proposed. P. Debye and W.F. Giauque independently made the proposal that lower temperatures could be reached by using the magnetic disorder entropy of electronic mag­netic moments in paramagnetic salts, a method later called adiabatic de­magnetization of paramagnetic salts. Not until seven years later, in 1933, was this proposal converted into a practical realization, when W.F. Giauque and D.P. MacDougall (Berkeley) reached 0.53 K, and a little later W.J. de Haas, E.C. Wiersma and H.A. Kramers (Leiden) reached 0.27 K. In fact, this was the first important low-temperature experiment since the end of the 19th century in which the University in Leiden was not the first to a­chieve success (for this early work see the relevant references in [9.1–3]).