The values β=0.395(10), γ=1.345(10), δ=4.35(6) for the asymptotic critical exponents, μ(0)${\mathit{h}}_0$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{C}}$=1.35(10), ${\mathit{DJ}}_0^{\mathrm{\delta }}$/${\mathit{h}}_0$=1.20(55), ${\mathit{a}}_{\mathit{M}}^{\mathrm{-}}$/${\mathit{a}}_{\mathrm{\chi }}^{+}$=-0.19(6) for the universal ratios and the ratio ${\mathit{J}}_0$/${\mathit{J}}_{\mathit{S}}$(0)=1.70(16), involving asymptotic and correction-to-scaling amplitudes, have been deduced from the bulk magnetic polarization data in the critical region near the ferromagnetic (FM)-paramagnetic (PM) phase transition of polycrystalline Ni samples of different shapes through an elaborate data analysis. These values, though close to those predicted by the renormalization-group calculations for a three-dimensional isotropic short-range Heisenberg ferromagnet, are shifted towards the mean-field estimates. Such a shift is taken to be evidence for a crossover to the fixed point corresponding to isotropic long-range exchange interactions. In accordance with the theoretical expectations, nonanalytic corrections (originating from the nonlinear irrevelant scaling fields) to the singular behavior at ${\mathit{T}}_{\mathit{C}}$ (Curie point) dominate over the analytic ones (arising on account of the nonlinear relevant scaling fields) in the critical region but the reverse is true for T≫${\mathit{T}}_{\mathit{C}}$. Initial susceptibility follows the generalized Curie-Weiss law [Eq. (14) of the text with a${\mathrm{̃}}_{\mathrm{\chi }1}$=0] from ${\mathit{T}}_{\mathit{C}}$ to 1.4${\mathit{T}}_{\mathit{C}}$ and the Curie constant permits an accurate determination of the atomic moment in the PM state. Not all but only about 80% of the moments (spins) in Ni actually participate in the FM-PM phase transition.

• Received 12 January 1995

DOI:https://doi.org/10.1103/PhysRevB.51.12585