Temperatures below 1deg;

Magnetic Cooling

“…Making use of the concept of a relaxation time IT within which the spins take up their stable orientation after a change has taken place in H , these authors found that T varied with temperature according to the relation ~= 2 . 3 x lo-' exp (-T/0.035) sec within the range of measurement, 0.04" to 0.12"K. Interpreting the results of MacDougall and Giauque (1936) for gadolinium nitrobenzene sulphonate heptahydrate in a similar way, they find 7=5 x lo-' sec at 0 . 2 5 " ~.…”

“…The temperature change upon isentropic application of a magnetic field can be By obtaining a lattice of (iW, H ) curves at various entropies ranging between ' high ' known temperatures and low unknown temperatures, the latter may be determined from this relation. This is essentially the method proposed by Giauque (Giauque 1936, Giauque and MacDougall 1938. Since the value of the absolute temperature being determined is very much less than 1 3 ~, and the high known temperature is about 1 ' ~, the former enters as a small difference between two relatively large quantities.…”

“…When the splitting pattern is known, the task of writing down the partition function is fairly simple. T o illustrate the general principles of the method, one may take the historically interesting case of gadolinium nitrobenzene sulphonate, which was studied experimentally by Giauque and MacDougall (1933 a) and ILlacDougall and Giauque (1936) and discussed theoretically by Hebb and Purcell (1937). If one assumes a field of cubic symmetry, the 8 s level of the gadolinium ion splits into two doubly degenerate levels and one quadruply degenerate level lying between.?…”

“…H E temperature region below 1 ' ~ constitutes a wide and relatively new field for physical research and may only be entered, effectively, by means of the T process of ' magnetic cooling '. It was less than thirty years ago, in 1926, that Debye (1926) and Giauque (1927) suggested, independently of each other, that the reversible temperature effects connected with the magnetization of certain paramagnetic salts might be used for producing temperatures well below those obtainable by means of liquid helium. T h e first experiments with this method were carried out in 1933 (Giauque and MacDougall 1933) and since then magnetic cooling has become an established cryogenic technique, both for studying the properties of the paramagnetic salts used as cooling agents and for carrying out experiments on other substances in this new temperature region.…”