SECTION 10.2. THE MAGNETOCALORIC EFFECT 95
Consequently, when the field H is applied under isothermal conditions, the heat released
by the spin system is
The same amount of heat will be absorbed by the system during the demagnetization process
after the field has been switched off.
The consecutive steps carried out in the cooling process are illustrated by means of
Fig. 10.2.1. The field H is applied at the temperature when the paramagnetic system
is in good thermal contact with the high-temperature bath (path AB in the lower part of
Fig. 10.2.1). For instance, the thermal contact is in on-position when the space between
the paramagnetic system and the high-temperature bath (which may be liquid hydrogen or
liquid helium) is filled with helium gas. Subsequently, the paramagnetic system is thermally
isolated by pumping the helium gas away (heat switch in off-position). Then, the magnetic
field is also switched off. In the lower part of Fig. 10.2.1, this corresponds to path BC.
The temperature has now dropped to The process described above is employed for the
production of very low temperatures in the microkelvin range (Little, 1964).
References
Hofmann, J. A., Paskin, A., Tauer, K. J., and Weiss, R. J. (1956) J. Phys. Chem. Sol., 1,45.
Little, W. A. (1964) Progress in cryogenics, 4, 101.
Morrish, A. H. (1965) The physical principles of magnetism, New York: John Wiley and Sons.