First- and Higher-Order Magnetic Phase Transitions in Dysprosium Aluminum Garnet

Abstract

Dysprosium aluminum garnet is a highly anisotropic (Ising-like) antiferromagnet which, in an applied field, undergoes a transition to the paramagnetic state without spin flopping. To investigate the nature of this transition we have made high-resolution measurements of the isothermal magnetization M and we have measured the specific heat CH as a function of temperature in different constant magnetic fields up to 14 kOe. The results show that for temperatures between 1.66°K and 1.14°K (and probably below) the transition is of first order, with dM/dH accurately constant and equal to 1/D, where D is the demagnetizing factor. The latent heat corresponding to this first-order phase change was calculated from the Clausius-Clapeyron equation and compared with direct measurements of the isothermal heat of transformation. Good agreement was found. From 1.66°K up to the Néel point (TN=2.53°K) the transition is of higher than first order with a point of inflection in the M(H) isotherms and slightly rounded λ-type peaks in the CH(T) curves. Close to TN the phase boundary is found to fit Hcint=A(1−T/TN)n, with n=0.50±0.02 and A=6.52 kOe. Our results show that the theory of a simple Ising model with nearest-neighbor interactions is only partially successful in predicting the behavior of a real material which also has magnetic dipole interactions.