ASYMPTOTIC CRITICAL-BEHAVIOR OF NI

The values β=0.395(10), γ=1.345(10), δ=4.35(6) for the asymptotic critical exponents, μ(0)h0/kBTC=1.35(10), DJ0δ/h0=1.20(55), aM-/aχ+=-0.19(6) for the universal ratios and the ratio J0/JS(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 TC (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 TTC. Initial susceptibility follows the generalized Curie-Weiss law [Eq. (14) of the text with aχ1=0] from TC to 1.4TC 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. © 1995 The American Physical Society.