Critical behavior of a site-diluted three-dimensional Ising magnet

The critical behavior of the site-diluted three-dimensional Ising antiferromagnet ${\mathrm{Fe}}_{1-x}{\mathrm{Zn}}_x{\mathrm{F}}_2$ has been studied with the use of neutron scattering and linear birefringence techniques. The neutron scattering measurements were performed on a crystal with $x\simeq 0.5$ and $T_N=42.50\mathrm{}$ K. The critical scattering has a Lorentzian profile both above and below $T_N$. The staggered susceptibility and correlation length exhibit power-law divergences over the reduced-temperature range ${10}^{-1\mathrm{}}$ to 2×${10}^{-3\mathrm{}}$ with critical exponents $\gamma ={\gamma }^{\prime }=1.44\mathrm{}\pm 0.06$ and $\nu ={\nu }^{\prime }=0.73\mathrm{}\pm 0.03$, respectively. From the linear birefringence $\Delta n$, the critical behavior of the magnetic heat capacity was determined on a crystal with $x=0.4\mathrm{}$ and $T_N=47.05\mathrm{}$ K. $\frac{d\left(\Delta n\right)}{\mathrm{dT}}$ clearly exhibits cusplike behavior rather than a power-law divergence over the reduced-temperature range 2×${10}^{-2\mathrm{}}$ to ${10}^{-3\mathrm{}}$ with exponent $\alpha ={\alpha }^{\prime }=-0.09\mathrm{}\pm 0.03$. The correlation-length and heat-capacity critical indices satisfy the hyperscaling relation $3\nu +\alpha -2=0\mathrm{}$ to within the experimental errors. Since all measured exponents differ markedly from the pure Ising values, this strongly suggests that random -exchange Ising behavior has been observed. The results are consistent with recent theoretical predictions for the random-exchange Ising fixed point by Newman and Riedel.