The Magnetic Refrigerator for Temperatures below 1 k

The magnetic refrigerator is a device for maintaining isothermally a reservoir and/or experimental apparatus at temperatures below 1°k. Starting from temperatures maintained by a liquid helium bath the device produces cold by the magnetic cooling method, the cooling being performed in a cyclic manner. By cyclic operation temperatures below 1°k, as low as approximately 0.2°k, can be maintained continuously. In this paper some design problems are considered in detail and a comparison is made between the theoretically computed performance and the experimental observations on the first few magnetic refrigerators which have so far been constructed. The `engine' of the magnetic refrigerator basically requires, besides a paramagnetic working substance for the magnetic cooling process, thermal valves which may allow or block the flow of heat between the working substance, the helium bath and the low temperature reservoir. In the work reported in this paper the material for the working substance has been ferric alum and the thermal valves have been constructed from superconducting lead wires. A discussion is given of the requirements for an ideal working substance and experimental results are reported on some unusual paramagnetic materials which have been investigated for use both as working substances and for reservoir materials. Amongst these ferric acetylacetonate and synthetic ruby have been found to show useful properties. Finally, some of the possible uses of the magnetic refrigerator are considered, as for example in calorimetry below 1°k, for the establishment of an isothermal reservoir for the initiation of single-stage magnetic cooling, for the production of liquid helium baths below 1°k and for the possible subsequent extension of the magnetic refrigerator to a two-stage device for the production of isothermal temperatures well below 0 1 k.