Asymptotic critical behavior of Ni

Abstract

The values β =0.395(10), γ =1.345(10), δ =4.35(6) for the asymptotic critical exponents, μ(0)h₀/k_BT_C=1.35(10), DJ₀^δ/h₀=1.20(55), a_M-/a_X ⁺=-0.19(6) for the universal ratios and the ratio J₀/J_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 T_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»T_C. Initial susceptibility follows the generalized Curie-Weiss law [Eq. (14) of the text with a^~_χ1=0] from T_C to 1.4T_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.