Critical behavior of the site random Ising antiferromagnetMn1−xZnxF2

Nuclear magnetic resonance has been used to measure $\beta$, the critical exponent of the order parameter, in the random Ising antiferromagnet ${\mathrm{Mn}}_{0.864}$ ${\mathrm{Zn}}_{0.136}$ ${\mathrm{F}}_2$. A single-power-law fit to the measured hyperfine field of the form ${\mathrm{H}}_{\mathrm{hf}}=B{t}^{\beta }$ yields $\beta =0.349\mathrm{}\left(8\right)\mathrm{}$ over the reduced temperature range of 5 × ${10}^{-4\mathrm{}}$ to 2 × ${10}^{-2\mathrm{}}$. This differs from the value $\beta =0.335\mathrm{}\left(5\right)\mathrm{}$ obtained by Heller [P. Heller, Phys. Rev. 146, 403 (1966)] for pure Mn${\mathrm{F}}_2$ and is consistent with the prediction that random disorder will cause a crossing to a new fixed point in the three-dimensional Ising system. The measured exponent can be explained in terms of recent predictions for the location and extent of the crossover region in reduced temperature.