Orientation of nuclear spins in metals

Abstract

In recent years, temperatures down to about 1/100-1/1000 degree absolute have been reached by means of the magnetic cooling method. This method, which was proposed by Debye and by Giauque, consists essentially of isothermal magnetization of a paramagnetic substance, followed by adiabatic demagnetization. Kürti and Simon (1935) have shown that the lowest temperatures which can be reached by this method are proportional to the interaction energy between magnetic dipoles of the substances used. Therefore, in order to reach still lower temperatures, it was proposed by these authors and by Gorter (1934) that the magnetism of atomic nuclei should be used. F. Simon (1939) has recently discussed this idea in greater detail and found that it should be possible to realize it experimentally. The question now arises of what temperatures can be reached in this way, what times will be necessary and what will be the properties of the nuclear spins at these temperatures. * In this paper we shall investigate these questions for metals. Here, the magnetic interaction between the conduction electrons and the nuclei leads to an indirect coupling between the magnetic moments of the nuclei which for many metals is considerably larger than their direct magnetic interaction. We shall show that any metal for which this is the case becomes ferromagnetic with respect to the nuclei. The Curie temperature will be of the order 10 -6 degree or less, and the temperatures reached by the magnetic cooling method will consequently be of the same order of magnitude. The attainment of such temperatures where the nuclear spins are orientated may be of great importance in nuclear physics.