Magnetic Properties of the Antiferromagnet DyPO4in Applied Fields

The magnetization and susceptibility of the Ising antiferromagnet DyP${\mathrm{O}}_4$ have been measured as a function of temperature and applied field for samples having demagnetization factors $D\approx 0.02$ and $D\approx 1.0$. Also, the temperature at which peaks in the heat capacity at constant applied field occurred were measured for a number of applied fields. These results have been compared with predictions of the molecular-field approximation and the Bethe-Peierls approximation including long-range dipolar interactions. The temperature below which the antiferromagnetic-paramagnetic transition is first order has been obtained within the molecular-field approximation. In addition, expressions are obtained for the magnitude of the discontinuity in the heat capacity at the transition between the intermediate (or mixed) state and the antiferromagnetic or paramagnetic states for samples having nonzero demagnetization factors. These expressions do not depend on the molecular-field or Bethe-Peierls approximations.