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 C_H 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 (T_N=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 C_H(T) curves. Close to T_N the phase boundary is found to fit H_c^int=A(1−T/T_N)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.