Abstract
Magnetic cooling experiments on a spherical single crystal of cobalt ammonium sulphate are described. Measurements were made as functions of the absolute temperature between 1° K and 0·05° K of (i) the specific heat, (ii) the principal magnetic susceptibilities, and (iii) the variations in susceptibility on applying (adiabatically) a steady parallel magnetic field. The measurements in the range from 1° K down to the Curie point are discussed in connexion with the dipole-dipole, exchange and nuclear interactions in the crystal. Below the Curie point, for the
K
1
axis only, an interesting effect was found, in that a steady field was observed to produce an increase in a. c. susceptibility instead of the usual decrease. At these temperatures the susceptibility was found to reach a fairly sharp maximum in a field of a few hundred gauss, and reasons are given for supposing that these maxima (more exactly, the minima in the curves of constant magnetization) mark the boundary of the antiferromagnetic region. The experimental results are discussed in connexion with Sauer & Temperley’s model for the antiferromagnetic state at low temperatures. A possible explanation of the apparent absence of a first-order transition at lower temperatures, expected in the theory but not observed in practice, is also given.
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