Magnetic Cooling

After a historical introduction the general principles of the magnetic cooling method are outlined. First, the case of the ideal paramagnetic salt is treated, and this is followed by a discussion of the electric and magnetic interaction effects which cause departures from ideal behaviour, and set a lower limit to the temperatures which can be reached upon adiabatic demagnetization. A description of experimental techniques and procedures includes the subjects of cryostat and magnet design. The methods which have been used to establish the absolute temperature in the range below 1 °K are surveyed and a summary of the results obtained is presented. Apart from their usefulness as coolants and thermometers, paramagnetic salts have intrinsically interesting thermal and magnetic properties, e.g. specific heat anomalies and hysteresis effects, associated with the internal ordering which proceeds as the temperature approaches the absolute zero. These properties are discussed in a separate section; another deals with the properties of other substances in the region below 1 °K - mainly metals and liquid helium. A section is also included on the difficult problems associated with providing adequate thermal insulation for a low temperature system on the one hand, and thermal contact within such a system on the other. The comparatively new developments of cascade demagnetizations and continuous cooling cycles are described. Finally, the principles involved in a recent striking application of the magnetic cooling method, viz. the spatial orientation of atomic nuclei, are outlined.