Random-Field Effects on Critical Behavior of Diluted Ising Antiferromagnets

Random-field effects have been studied, using linear birefringence, in the random Ising antiferromagnets ${\mathrm{Fe}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2$ and ${\mathrm{Mn}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2$. with $0<\mathrm{}{H}_0<~20$ kOe parallel to $c$, large shifts in $T_c$ and dramatic changes in the shapes of the transition peaks were observed. These results confirm some, but not all, theoretical predictions and suggest an effective dimensionality $d\simeq 2$. No $H_0$-induced rounding was observed. This is shown to be consistent with the rounding seen in other experiments at larger values of $\frac{g{\mu }_{\mathrm{B}}{H}_0}{k{T}_c}$.