Time effects in the magnetic cooling method─I

Abstract The recent progress in attaining extremely low temperatures was made possible by the magnetic method of cooling. This method is based on the following principle: Supposing we have a paramagnetic salt, the ions of which carry a magnetic moment (spin) μ. We assume that the interaction of these spins with the lattice and with one another is so small that the spins can be considered as relatively free. These properties are realized in some salts of the rare earths and of the iron group. In the absence of a magnetic field the spins are distributed at random. The crystal has then a comparatively large entropy due to the exchange of spin directions. If we now switch on a magnetic field H isothermally, the spins will be orientated in the direction of the field. The degree of orientation will depend upon the ratio Hμ/kTi, where Ti is the initial temperature. The entropy will decrease according to the degree of orientation produced by the magnetic field. The decrease in entropy is a certain function of Hμ/kTi.