Critical exponentγfor Ni, as measured by perturbed angular correlations inNiRh100

A review of theoretical predictions for the critical exponent $\gamma$ in the $d=3\mathrm{}$ Heisenberg model indicates that $\gamma ={1.38}_{-0.01\mathrm{}}^{+0.02\mathrm{}}$. Available measurements on Ni, all using the bulk technique, show a range of values between 1.28 and 1.35, with the best measurement falling at $\gamma =1.31\mathrm{}\pm 0.01$. In order to investigate this apparent discrepancy between theory and experiment, we have done a new measurement of $\gamma$ via perturbed angular correlations, using $\mathrm{Ni}{}_{}{}^{100}{}_{}{}^{}\mathrm{Rh}$. Because of the great sensitivity of this method, we have achieved better definition of $T_c$ and lower applied fields than in previous work. Using both asymptotic and scaling equation of state analysis, we confirm that $\gamma =1.31\mathrm{}\pm 0.01$. We conclude that the critical behavior of Ni can not be described by the isotropic Heisenberg Hamiltonian.