A new method of correlating the magnetic temperature scales of paramagnetic salts below 1° K

By making use of the extremely anisotropic susceptibility of cerium magnesium nitrate to provide a magnetic thermometer, the susceptibilities of several paramagnetic salts have been determined at temperatures below 1° K. The measurements have been made on specimens in the form of a spherical shell of the substance to be examined enclosing a single crystal sphere of cerium magnesium nitrate. By measurements on the compound specimen in directions parallel and perpendicular to the magnetic axis of the cerium crystal, the susceptibilities of the two components can be separately determined. Since that of the cerium salt obeys Curie's law accurately, the temperature is readily found, and hence the susceptibility-temperature relation for the material of the shell. The problem of thermal contact between the components has been investigated, and it is shown that reliable results can be obtained at temperatures down to 0.05° K. The demagnetizing effects in a concentric spherical specimen have been calculated, and all the results are expressed in terms of the magnetic temperature defined from the inverse of the susceptibility of a specimen in the form of a solid sphere. Measurements have been made on neodymium magnesium nitrate, three cupric Tutton salts, ferric ammonium alum, and manganous ammonium sulphate. The results obtained with neodymium magnesium nitrate are compared with the behaviour predicted theoretically, and good agreement is found.