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

Abstract

The critical behavior of the site-diluted three-dimensional Ising antiferromagnet ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{F}}_{2}$ has been studied with the use of neutron scattering and linear birefringence techniques. The neutron scattering measurements were performed on a crystal with $x\ensuremath{\simeq}0.5$ and ${T}_{N}=42.50$ 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 ${10}^{\ensuremath{-}1}$ to 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ with critical exponents $\ensuremath{\gamma}={\ensuremath{\gamma}}^{\ensuremath{'}}=1.44\ifmmode\pm\else\textpm\fi{}0.06$ and $\ensuremath{\nu}={\ensuremath{\nu}}^{\ensuremath{'}}=0.73\ifmmode\pm\else\textpm\fi{}0.03$, respectively. From the linear birefringence $\ensuremath{\Delta}n$, the critical behavior of the magnetic heat capacity was determined on a crystal with $x=0.4$ and ${T}_{N}=47.05$ K. $\frac{d(\ensuremath{\Delta}n)}{\mathrm{dT}}$ clearly exhibits cusplike behavior rather than a power-law divergence over the reduced-temperature range 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ to ${10}^{\ensuremath{-}3}$ with exponent $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=\ensuremath{-}0.09\ifmmode\pm\else\textpm\fi{}0.03$. The correlation-length and heat-capacity critical indices satisfy the hyperscaling relation $3\ensuremath{\nu}+\ensuremath{\alpha}\ensuremath{-}2=0$ 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.