Temperatures Below 1°K
Until the rare, light helium isotope, He3, became easily available in the early 1950’s the expression “temperatures below 1°K” was frequently used as a synonym for “cooling by adiabatic demagnetization.” There was some justification for this. It is true that even He4 permitted the attainment of temperatures somewhat below 1°K-the lowest seems to have been about 0.7°K-but this was rather the exception and a glance at the literature shows that the bulk of experiments using liquid He4 stopped near 1°K and work below this temperature relied almost entirely on the magnetic cooling method. He3 with its lower boiling point and higher vapor pressure has changed all this since quite a few He3 cryostats now operate down to 0.25°K. These cryostats are thus within the scope of this discussion. However, they will be treated rather briefly for the following reasons.
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