Magnetothermodynamics of Single Crystal CuSO4·5H2O. V. Fields Along the β Axis. Thermodynamic Temperature without Heat Introduction below 0.5°K. A Reference at 0.035°K

The magnetic and thermodynamic properties of a 3.700‐cm‐diam spherical single crystal of CuSO₄·5H₂O have been investigated down to 0.02°K with fields along the $\beta$ magnetic axis. Thirty‐eight magnetizations from unknown low temperatures to reference conditions were performed under adiabatic conditions, and 35 were essentially isentropes. These have been used to determine the thermodynamic temperature and heat capacity, both in and out of magnetic fields, without heat introduction. A temperature reference at 0.035°K has been suggested. It has been pointed out that proving a process to be isentropic is no guarantee that “frozen‐in” entropy and energy situations have not developed during the isentropic process, thus conserving nonequilibrium entropy. Entropy conserved in this way cannot be used as an equilibrium variable in determining temperature. It is evident that detailed methods of investigation designed to detect such nonequilibrium magnetic structural situations will be desirable. When “frozen‐in” entropy situations develop on proven isentropes, as is inevitable in many cases, the isentropic transfer of the entropy values to unknown low temperatures will not suffice to determine the thermodynamic temperature by use of heat introduction and the equation $\mathrm{T\hspace{0.17em}=\hspace{0.17em}dq\hspace{0.17em}/\hspace{0.17em}dS}$ . Earlier magnetothermodynamic data have been supplemented by measurements between 0.4 and 4.2°K with a field of 33 kG.